This is a "real" vortec CPI. Developed a slight misfire at cruising speed that has gotten much worse over a couple monthes. As a "first action", I replaced the:
original wires
plugs
Cap and rotor
Fuel filter.

Made zero difference.

Symptoms:
It does it MOSTLY between 2k and 3k RPM, at about 1/4" throttle (~20% on a scan tool). At that RPM it's terrible, bucking hard (almost like it's "kicking" from too much spark advance), and it also backfires through the exhaust. Basically cruising at a constant speed on the highway, it's terrible. The misfire is totally random and sparatic.

It idles perfectly, and anything above 1/3 throttle, it runs perfectly all the way to 5500 RPM, WOT, it pull very very hard, for what it is. So if the R's are below 1900 or above 3000 it's not bad, OR if the gas pedal is 1/8" or less or 1/3 or more, it doesn't do it at all.

I run only Mobil 1, Chevron gas (to keep the CPI injectors clean) and it has good compression and fuel pressure. When the symptom occurs, I get a "random multiple cylinder misfire" code and that's it.

 

Any suggestions here?...

These syptoms are driving me crazy.

-Tom

 

At the risk of sounding stupid, what's CPI?

 

CPI= Central Port Injection

 

CPI stands for Central Port Injection. It's a kind of cheesy SFI system GM made for 4 years where all 8 injectors are clustered in the center of the intake, INSIDE the plenum. There is no conventional "fuel rail", just a feed and return line to the cluster. The FPR is also located inside the plenum, and is "ported" to manifold vacuum (like any other Port FI system), but by virtue of the fact that it's housed in the vacuum that it's refferenced to. A single harness plugs into the injector cluster to drive the 8 injectors sequentially. Since all the injectors are clustered together in a clump in the middle of the plenum, to get "port" injection form this wacky set up, there is a hard plastic line running from each injector, to the standard "port" injector location in the intake runner (cross-ram style intake), where the fuel is sprayed into the port at the intake valve. To keep fuel from "dribbling" out of this hard line between injection pulses, there is a poppet valve at the end of the hard line, in the intake runner. This valve requires at least 45 PSI to open, so it only opens when the injector fires, exposing the poppet to 55+ PSI fuels system pressure (Assuming everything is workng properly). This is how the system maintains fuel in the hard line between injector firings.

That's about it. In a nutshell, it's basically a more modern, "Sequential multiport Crossfire Injection"! So "Central" because all the injectors are grouped into a simple, cheap unit, and "port", because they use hardlines to get that injection to actually occur at the standard port injection location.

 

check the throttle position sensor a s it travels thru that critical throttle opening. If the voltage jumps around asyou open the throttle....you know what to do.

 

is this one of those designs thta does batch for sequintial firing? if it is the sequintial is there a chance that at that rpm range the injectors might be far enough out of time to cause some weird issues? well at least if the injectors . granted though only way I can see that going on is if the hoses going to each injector are off sequence.

maybe some sort of gunk in the injector lines.

 

That setup is junk, and you might have a few bad injectors. It is a known problem with GM that the poppet injectors get stuck, plugged, or whatever and run like pooh. For some vehicles there is a replacement spider web of injectors that converts them to regular electrical injectors instead of the poppet style.

 

F-Bird,
I did watch the TPS reading on the scan tool carefully while driving (and also watching the road). While the TPS was reading AROUND 20%, it would do the syptom, but the TPS reading on the scan tool didn't move. So I figured it was fine. HOWEVER, the more I drive it, the more I'm convinced that it's the TPS, just because it does at that one spot in the throttle range SO repeatedly, and also that is the spot where 99% of my driving takes place. Thus, the most wear on that part of the TPS. I'll test it manually. Thanks for backing up my "feeling" there.

RX-7, It's a true sequential system. It just places the injectors at remote location from the port.

Lo-Tec, The system isn't junk; it IS cheap-O, but it's not junk. It's been functioning flawlwessly and utterly maintenance free for 161,000 miles to date. It starts on 1 compression stroke (1/4 revolution) hot or cold, and returns OVER 20 mpg highway...in a 4500 lb+ 4wd vehicle. IMO, that's not junk. As for injectors sticking, you're right. IF you run garbage gas in them, they'll stick. However I have delt w/this in the past (not on my vehicle), and the symptom when that occurs is a total out right and consistent misfire of the one (or more) cylinder(s) that the poppet is stuck on, just like a plug wire that's removed: It won't fire again untill you unstick it. I've learned that the easiest way to unstick the injector (poppet) is to remove the plenum lid, and tap on the poppet while cranking the motor. As soon as the poppet starts spraying fuel, it's good to go again. That's not my problem though, as my engine doesn't exhibit those symptoms. It misfires in one tiny operating perameter, and it does so totally randomly and wildly.

I'll manually check the TPS and report back...

-Tom

 

Well, it wasn't the TPS.

I tested it w/my DVOM, and it seemed to check out fine. This is where an Analog VOM is way better, IMO. Anyway just for yuk-yuk's, I swapped TPS's w/one of the vehicles from work (we have 30 w/the same engine). NO difference! Driving home it was actually probably about as bad as it's ever been. It was throwing a complete HISSY-FIT in 4th gear on the highway. Here is what the scan tool "froze" when it threw the code:
Trouble code:----P0300 (Random multiple Cyl misfire)
RPM:------------2566
Calulated Load:--27.5%
MAF (lb/m):------5.72
MAP (in HG):-----11.8
Coolant:---------194*F
LT FTRM1%:-----3.9
ST FTRM1%:-----0.8
LT FTRM2%:-----0.8
ST FTRM2%:-----3.1
Speed:----------68mph
Fuel System1:---Closed
Fuel System2:---Closed

I have no idea what "LT FTRM" and "ST FTRM" mean or what the data following them means either. Anyway, I'm at a loss here. The fact that it runs so good above 30% TPS, and I mean SO GOOD, I just can't see how it could be a basic ignition or fuel issue. If it was, it should show up there first (under load). Then at idle next probably. Everything APPEARS to be functioning fine to me, even in the freeze frame above while it's running like sheitza. I don't know where to move next...

-Tom

 

is the system maf or map? could it possibly be that the map sensor is wigging out on you? at least if it was map that could make sense. if it is giving some erroneous readings only during such a load.

also what about the EGR valve? those things can cause some weird issues when opening or not opening at the right rpms. or if the valve was leaking when it opened up it could cause some issues as well I'm sure.

guess one thing you might be able to try to do with your multimeter is read the voltage to the map while cruising see if when using smooth throttle transitions if the voltage on the map follows suit or if it reads weird.

same thing maybe with the EGR if it is electricly opened is see if the EGR opens at the time your readings are taking place.

not sure if this would help or if there are easier ways I'm just throwing some ideas out at least
the FTRM numbers look like it would stand for fuel trim. ST and LT look like long term or short term fuel trim. I assume it's just the computers way of "learning" based upon how the car is running with readings from the O2 sensor. but again that is just an assumption

 

Both. It uses a MAF sensor in the intake, AND a MAP sensor on the plenum. Your idea makes sense, but I can sit and read the MAP reading while I'm driving down the road and it appears to follow the readings I think it should. Smooth, consistant...seems fine to me.

This is a possibility. I do get an "EGR Pintle position" code now and then...but not when I get the P0300 code. (?) When I'm driving, I can't see anything about the EGR on the scan tool that I have, so I don't really know what it's doing.


That seems logical. I wonder what the numbers mean if that's the case.

 

tom, my 2000 astro does something similar. i've been fighting with it for the last 4k miles.

only, it does it when it's damp or rainy outside now.

the first time, the cap looked like the terminals were corroded. replaced cap, rotor, plugs (broke a couple while removing) and wires (a connector snapped on me). that allegedly cured a P0301.

now it just throws a P0300 whenever it's wet or rainy outside.

only thing left i need to change is the fuel filter.

:shrug:

 

That's interesting. We should stay in touch about this.

Mine has no corrilation w/wet weather at all. As a matter of fact, yesterday was crystal clear, sunny and dry. And in Park City Utah, dry is DRY! Yet it was still throwing a tantrum. Yours really sounds like a sendary ignition issue, w/the corrilation to wet weather, IMO. But you've covered your basics by replacing the cap, rotor, wires, etc.

How many miles does your Astro have? I'm seriously wondering if there is some sort of whacky harmonic thing going on w/the distributor at that RPM. More experimentation last night showed me that it's the WORST at right around 2600 RPM, AND 18% throttle. I'm going to un plug the EGR while it's idling then drive it to work this AM and see if that makes any difference.

-Tom

 

could you find out what wire it was that powered the EGR? or is there a way to backprobe the ecu if it has a ecu turn on wire? another thing with the EGR depending on how it is located is it might be externally leaking as well rather then just oepning or closing at the wrong time. if nothing else maybe block the EGR off? or disable it from working.

do you have a different ECM to try maybe?

as far as the fuel trim I would assume it's how much it's taking out or adding of the default pulse width table.

 

What you really need to check the TPS is a scope. Even a DVOM won't notice if there is a sudden dip involtage, it's refresh rate isn't quick enough. But if you use a scope you can look at the patten and actually see a sudden dip in voltage.

I was confused for a while though, CPI refers to central port injection on the old 92-95 vortec 4.3 V6s. CSFI is what is only the vortec V8s. CPI is not sequential, CSFI is. It's Central Sequential Fuel Injection. Either one makes a very good induction system. I had CPI on an S-10 and loved it.

 

RX,
The EGR is controled by a stepper motor. The EGR unit is the motor, valve and the pintle position sensor all in one. This morning, I disabled it (unplugged) and drove to work. Verdict? The truck ran flawlessly. Except it did throw a "EGR pintle position" code of course. So I'm going to try swapping the EGR valve assy and see what happens. I don't know why or how the EGR could cause the symptoms that it did, other than if it's just plain dumping way too much EG into the intake (?). But that seems to be where the problem is. I'll keep you posted.

I don't need a scope. I tried a known good TPS and it made no difference.

'92 ('88 actually) thru '95 4.3 "Vortec" engines used TBI. It was called "TBI". "CPI" was used on the 4.3, 5.0, and 5.7 from '96 -'99 and it WAS sequential port injection, w/the injectors located in a central pod. Just as I described above and "Toehead" said as well. I've never heard of a "CSFI" system. You might want to check on that one and get back to us...

-Tom

 

DLV555 is right about the CSFI nomenclature. My 97 truck is 5.7l CSFI, and the CPI was for the 4.3 V-6 engines. they had one single injector with poppet valves. or, I should say, becuase I'm not sure, there was one injector block, that is serviced as one peice.

the v-8 96-98(?) CSFI is sequential and central port. there is a company that sells individual injectors. lets see.....try this link:

Home


not the link i was thinking of, but its a decent link for some info.

 

Huh. Where are you getting your info? I have factory (GM) sevice manuals here at work and they refer to all of the above as "CPI". Either way the 4.3 still uses an injector "pod" w/individual injectors for each cylinder, but located in a central cluster.

-Tom

 

Oh I didn't realize you had a spare TPS, then you were right you don't need a scope.

I have to disagree, the 4.3L CPI engines (92-95) had the centrally located injector assembly, but it was only one injector and it fed all 6 poppet valves (that funky spider looking thing). The CSFI system is the one that had one injector per cylinder, was fired sequentially, but still had a centrally located injector pod. CSFI manifolds were much different with a cental downdraft throttle body, that I routinely confused for TBI before I knew anything about it. From what I can gather "CPI" is sometimes used to describe CSFI, though they are technically two different things. To my knowledge Vortec engines did not use TBI, but I can easily be wrong.

Picutre 1 is a CPI diagram, picutre 2 is a CSFI diagram.

 

You're right. I was wrong.

I forgot about that interim system they used there for a couple years between TBI and CSFI. Sorry about the misinformation on my part.

-Tom

 

This is going to sound like garbage, but have a look.

A few years ago my T/A had very similiar symptoms. Like you guys I was looking around the TPS, EGR ect ect. So what was it......A bloody vaccum leak.

Yep thats right, actually a few of them. The engine wasn't compensating enough for the leak / low vaccum until the throttle was further towards open. The thing was, the leak was not large enough to set any code that might lead you to that line of thinking, but large enough to do the above.

I couldn't believe it.

 

I don't think that's garbage, and I actually considered it, but it doens't do anything abnormal at idle. And that's usually where vacuum leaks show up the most.

Did you car idle fine? Were the symptoms completely the same?
That could explain why it's gotten progressively worse...

-Tom

 

tom, my van's closing in on 116,000 miles.

i'm actually going to check the wires again and maybe look at some better replacements.

 

For the reasons you just said above I didn't consider it. It was fine at idle. Symptoms seem fairly close from memory, it was about 4yrs ago.

 

could be as you said too much EGR in the intake another thing depending on how your EGR is setup but it could also be leaking externally so when the EGR opens up at that point it just creates a vacuum leak. and with EGR usually only working under light load cruise conditions either one could make sense.

EGR is nice when it works when it doesn't they cause all kinds of weird things even more so being they don't open all the time just under the right conditions. idle they are fine cause they are off. WOT they are fine cause they are off (or at least should be for either one of those) under light load they could be either way on or off.

 

Well, it's not the EGR either.

I replaced that last night, drove home and same thing. I'm betting on a vacuum leak at this point. Gotta get out the break clean and start spraying it around all over the intake. Will report back.
-Tom

 

it would still almost seem EGR related though if by disabling the EGR makes it run fine.

 

Well now I'm just as lost as ever. I sprayed brake cleaner all around the intake where the upper and lower meet, around the injector pod, and also around every point where something penetrates the intake (like the vacuum port for the brake booster for example). There was no effect to the engines running at all. Just to make sure the brake clean would work for what I was trying, I pulled the PCV valve our of the valve cleaner and put a quick shot in there, and it stalled the engine.

So, from what I can tell, there is no vacuum leak. Once again, I'm out of ideas.

-Tom

 

How does it run with the new EGR valve disconnected?

 

Same as the old one; Perfect. Except that it throws and "EGR Pintle Position" code of course.

-Tom

 

Odds are that the egr passages are all carboned up, or there is some kind of internal leak related to the egr (unsure of what kind of leak, but this is related to the egr).

I had this problem on my 95 caddy, although it didn't misfire like yours, just pinged lightly all the time while rolling down the highway. I used some GM spray top-engine cleaner to clean out the egr passages, and threw on a new egr valve b/c the old one was original and already off the car. Fixed the problem 100%.

 

Tom, it's a known problem with the injectors. I hate to say it, but you're going to end up there. Let us know.

 

If you do end up needing injectors, ditch the current setup underneath and go straight for the GM MPFI retrofit ones. They eliminated the poppet and put injectors at the end of each nylon tube. This change was done around 2001 or 2002 in the express lineup.

 

Fast 355,

That's some good info. I'll look into that.

I REALLY don't think it's the injectors. Why?
1. We have 30 trucks w/these engines at work, and I know what the symptoms have been in those when there are injector issues. These symptoms are different.
2. When you unplug the EGR, it runs great. Bad injectors would most likely continue to run badly, IMO.

I talked to a tech at our local Chevy dealer today and after hearing the symptoms and what I've changed so far, he was stumped too, but agreed w/me that the symptoms don't point toward injectors. "You've got a good one (problem) there"...he said. Sweet.

HOWEVER, Because I have NO IDEA what else to persue, I'm going to swap out the entire injector/spider assy w/a truck from work, and see if that moves the problem (to the other truck). What ever happens, I'll report back, then if it is the injectors, I'll persue what F355 said.

Thanks to all for the continued input and support.

-Tom

 

Hi, I just came across this thread and wanna give some input. My tach does jump around 2000rpm. however, the car runs fine. (This happened after I installed A CD ignition box). So, I'd say you should check your ignition and see if everything is working like it should.

If the EGR when plugged, makes the car run good, than I would just advise you run the car without EGR untill you figure out what exactly is the problem. You should be thankfull that you narrowed it down to EGR! Now do some elimination stuff and you will find what the heck is going on.


The vacum lines that go to EGR might be cracked or so, but since you unplugged them and it worked fine, it eliminates the vacum lines.
Gasket?? Maybe, but it probably isnt.
The internal passages?? Probably, because when EGR returns too much exhaust gas back it will make the motor run poorly.

Like I said before just disconect it till you find the problem.

Laters.

 

Any update on this thread? My Vortec is doing similar things and would like to learn what happened.

Thanks!

 

Well only half the ignition system was diagnosed. Need to look at the speed timing pick up in the timing cover, also look at the scan tool and see if you are getting considerable knock retard, this could be cause by a faulty or short knock sensor.

 

update:

sammy, i haven't been driving my van much at all recently.

the rear axle blew up in june, and it sat for the better part of the year. i got the van back on the road in october, only to discover that the transmission is on its way out.

i haven't made time to diagnose the misfiring issues. gotta put a transmission in it now. :shrug:

 

Thanks robertg.

My axles are still doing quite well but I had to put a Transgo fix shift kit in my transmission 120,000 miles ago. Mine still looks fairly nice and I sure would like to make it run nice too. Well actually I remember now my front axle has a small pinion seal leak.

 

My update.

I've basically been running it since my last post w/the EGR valve unplugged, and it's continued to run great, and get awesome gas milage, for what it is.

Again, the check engine light is on all the time, which is annoying, and it's on because it's seeing an erronous EGR position (open).

This system uses a pipe running from the drivers side exh manifold to the EGR at the front of the intake. I wondered if that pipe had rotted out (it's wrapped w/heat cloth like header wrap so you can't see it). I removed that pipe and not only is it clean/clear (I can blow through it), it's also sound; it has no holes or leaks.

At one point, I plugged the EGR back in, and it ran good for about a day, then the symptom came back and got bad again, so I unplugged it again. I'd really like to solve this issue.

I know Red Devil thinks it's the injectors, but I disagree. Even if he's right, there's no way I'm laying down $350+ on a "guess". I need to be able to diagnose before I spend that kind of money.

 

How do I test the "speed timing pickup"? I've never even heard of that sensor (?) before. If it were that sensor, why only a small RPM and throttle range (the same range that the EGR is supposed to open)? I would think that any type of magnetic pick up would cause issues at any RPM and certainly wouldn't be related to throttle position in anyway.

I don't think it's the knock sensor. If it was, the symptoms wouldn't improve from disconnecting the EGR. In fact they'd likely worsen as it *should* ping worse w/the EGR unpluggged.

 

Well if the reluctor wheel that fires the sensor is damaged, it would only happen at a certain range. Also the sensor could very likely be damaged in a way that it does not read a certain frequency that is produced at those RPM.

You need to look at your timing when it is running. If it is the knock sensor. EGR may or may not improve the situation.Unplugging that does not really diagnose anything. Once you stop injecting exhaust gas to cool down the cylinder the engine is going to exhibit different behaviors. Depending on how agressive the timing is it could make the misfiring worse. Or if the timing is lazy a fresh air charge will pick things up.

 

That makes sense, but not when you re-read the symptoms. Notice that although it ONLY happens in a certain RPM range, it also REQUIRES a certain Throttle position. Meaning the RPM range isn't the only factor. 1/3 throttle or more, and it won't do it, period...at ANY RPM. If it was some thing like the reluctor wheel or pick up, vs. RPM, then it would misbehave at that "trouble" RPM at any throttle angle.

I'll check timing though, and post back.

 

I have a 96 K1500 with the 5.0 vortec motor. I was having similar issues with mine and I was playing the same parts replacement games with no luck. Then I found out that in 96(first year with the vortec 4L60E combo) the trans computer had a bad ground from one side to the other. This was causing the computer to get confused and not applying the trans pressure correctly which would make it surge and miss and do all kinds of engine type things that you wouldnt think would be trans related. There is a service buliten about it, pretty much you can either replace the computer for about $350 or buy the $32 wire kit which fixes the ground issue. I installed the wire kit and my truck was kickin *** again, better than when I first bought it. Everything was great for about 2 days and then some of the symptoms started coming back. Not sure if the wire kit fried(its just one wire) or what but I really want to get my truck running right again.

 

Interesting. Mine's a 5 speed though. Know if the same bulletin applies?

 

Not sure. It applies to those with the VCM (computer on drivers fender well) with serial number ending in 85 I believe, if it ends in 22 its the updated one and you dont need it. Im not sure if the manual trucks use the same box or not?

 

Try following the info in this thread, I have a 96 C/K 1500 with the 4.3 V6 and it has been doing this for about 4000 miles but just started throwing the P0300 code. I am going to go for the Bosch distributor cap and rotor and check the gear at the bottom of the distributor.

http://www.obdii.com/ubb/Forum1/HTML/001357.html

 

Hi Tork,

That link is wicked awesome! Thanks!!!

I especially like this guys post, as it's exactly the same symptoms as mine!:
Here it is:
I'll be checking my dizzy gear (and the timing).

 

After reading everyones excellent contributions, last night I worked on my 98 4.3L Astro. I had put ~195,000 on my original Delco cap and rotor. 15,000 miles ago I put a new brass NAPA cap and rotor on mine. I figured it should be good to go for a long time, but it had quite a bit of green corrosion on the posts and green tracing inside the cap. The distributor vent on mine was open and not clogged. I had already bought another NAPA brass cap (I think next time I will get the Delco one because I can't find a Bosch part number for the application?) so I went ahead and put it on.

Before I did that I felt the distributor to see how much backlash it had, how much endplay, and whether it felt like the top bushing was worn out. It seemed to have a lot of endplay (but I always thought that was normal on GM distrubutors?). There is some backlash, more than I would have liked to have felt but not total slop either. The top bushing seems nice and tight when pushing the rotor side to side.

After pulling the distro and looking at the gear, I see it shows signs of wear. The wear seemed to all be on one side, the driven side (like it didn't really show evidence of slapping back and forth). Not too bad but visible. I am sure the perfectionist would have immediately replaced it but I went ahead and slotted the hold down bolt hole anyway to see if I can get more life out of this one by reindexing it.

I put it back in and bolted it down ~ 3/16" counter clockwise as measured at the distro hold down bolt. Counter clockwise seemed to be the rotation that would make the spark pattern match up with the posts better.

I don't know what year Chevy went with a roller cam (I suppose making it steel) on this motor. Maybe they weren't up to speed yet on what material to make the distro gear out of. Even though the distributor itself spins really easy, that gear also has to run the oil pump. And anyone who has primed an engine with an electric drill knows how much torque that takes!

I think the next time I put another new cap on it (maybe another 15,000? :> ) I will drop in a rebuilt distro too.

Those caps aren't cheap either, $40 for a brass one.

Back in the days of centrifugal advance distros, the rotor would advance along with the timing to keep the rotor indexed better, now that it is all electronic, the rotor is static in relation to the advance and when the timing advances it forces the spark to jump a larger gap making it easier to misfire I guess.

I will take multiple days driving to see whether this helps. My light throttle misfire and P0300's would be worst on more humid days and days above freezing.