91 350 TPI.

Runs rough at lower rpms and feels like miss, same at idle. can feel it most from like 1300 -2000rpm, especially in park.

Fuel Pressure is 38 line on and 45 line off.

plugs, wires, coil, module new, distrib is from december.

Injectors are new and had access to test set to swap.....

Vaccuum at idle is 19.5inches and only varies with the miss by .5 inch, rpms go up it increase and steadies(depending on the current miss)

Gas mileage is horrible, car has no ***** either, feels sluggish.

No codes and when datalogging nothing appears amiss, map readings seem a little higher under cruise and lite load than previously.

Its been like this awhile too, was getting nearly 24mpg with the car loaded down. (fixed a bad plug wire and broken rubber line on the evap and ran awesome for about 1200 miles while i was home for break)About 1/2 way back to school I filled it up, added techron and the car went to hell.

I cannot pin it to any one cylinder either, pulling wires or injectors.

Its almost aways present, also does it when cold as well, but more intermittently, depending on tempature when started.

Idles at 600rpm in gear, with the miss.

Apears to be almost random like, but it is literally a constant type miss, unable to pin it down.

Only thing I am wondering, I do have some oil on the intake on the pass bank. The pass bank of plugs did look dirtier than the drvrs bank when I swapped them out. I pulled injectors on the drvrs side and watched the BLM and they all responded the same way, but since no o2 on the pass bank I cant test them as well.

Every inj lead has 12v at it with key on so they are good as well

Also pulled and blocked egr vacuum as well, no change.

Does same thing with FP vacuum pulled as well with the higher resulting pressure.

Any ideas, please read before u reply, not being mean, but someone is gonna ask me what the vaccuum is doing or my fuel pressure lol

thanks
Jeremy

 

a few thoughts off the top of my head.
have you checked the compression? dry & wet.
does it puff smoke from the crankcase with the PCV valve pulled & the breather hose blocked?
does it pull a vacuum in the crankcase at idle with the PCV valve pulled & both the PCV valve hole & breather hose blocked?
do you have access to a scope to check the firing voltage?


oh, whats the fuel pressure & vacuum look like?
sorry, i figured your right, someone will ask, plus i couldn't resist

 

The egr valve could be stuck open. First tap the egr valve while the car is idling. Did it smooth out? Then try to open the egr valve when at idle. Apply vacuum to the valve and see if the car stalls or runs rough. If the idle does not smooth out to what it was, then the valve is stuck open. Sometimes carbon keeps the egr valve cracked open or stuck open. It will cause a miss that can not be traced to one cylinder. GM top engine cleaner can get rid of the carbon build up.

 

No way to do compression easily.

Im borrowing jacks and tools since Im here for school lol

All ignition compenents are recent and re-tested repeatedly.

No smoke from the crankcase at all. I didnt check to see if its pulling a vaccuum in it however.

No access to a silly scope to check out the ignition either.

EGR wise, if it was stuck open it wouldnt be felt at light loads cause its supposed to be on then.

Weird thing is, I swore an injector wasnt working #1 plug was white and the rest were dirty/fouled looking(writing that off due to techron being added when I filled up right before it happened, hasnt done it since either). When I swapped one old one out for the new, and removed some of the other injectors and primed the pump a few times, it ran completely normal, no miss for about an hr when I went out after we put it back together................................

 

ohm your injectors, if they arent all almost exactly the same then its time for new ones. they should ohm out around 16

 

Might want to re-read what I has posted already

 

Are you still running a factory exhaust with cats, The way it sounds you may have a cat that has gotten clogged the random miss would be caused by the build up of pressure in the exaust system. The quick way to check this is to go to the tail pipe and feel the flow if it doesn't pulse then most lickly your cat is clogged.

 

Cats are new and old ones are good, regrettably sitting on the floor here.

 

I have a match for ya.

Check the fuel lines for restrictions. A couple local guys have/had problems with fuel lines clogging with some nasty whitish crystalline crud.

 

Im leaning that way mm.

Two things made me lean there.

Thought an injector from the new set was bad. Swapped it out and removed several others and car was fine for an hr or two.(primed the rail and blew it down with injectors pulled)

Second, yesterday I deadheaded it. From 42psi and one turn off the key not even a whole primce cycle it hit 100psi instantly.

This morning it seemed a little better cold starting and in closed loop.

Going to get a fuel filter and prolly pull the entire rail off next chance I get and disassemble it to see if I can see anything

later
Jeremy

 

All the ones I know of clogged at the FPR housing. Start there.

 

Really now?

Thats where my gut hunch was going.
Interesting, prolly gut it after school if I have enough time.

It also will tend to come and go when I play with the fuel side of things.

What kind of pressure were your friends seeing when all this was happening?

later
Jeremy

 

Your car sounds just like mine. Throughout the years I have replaced nearly everything on the car and it still isn't running like it should.

It is better than it ever has been right now, however on my best days I only squeeze out 15-18mpg.

 

I cant recall. I think it was pressure spikes on one.

 

I had the same symptoms after my re build. My O2 sensor got damp while the engine was out of the car. Might just want to make sure yours is in good working order.

 

Still runs like crap.

Cannot isolate to any one cylinder.

Snagged another distrib to test with and same crap happens. Actually seems worse with new distrib and it has a moving idle. Floats over a 200 rpm range or so.

Still feel/hear miss at idle and when revving in 1400-2k range.

Completely disassembled fuel system to make sure everything was clean and it was. Cleaned everything out(not much to clean) and put a new diapragm in the regulator also.

Im just lost and baffled.

I've also gone thru and unplugged every sensor one at a time to see if something changes and it doesnt.

It is worse hot but u can feel it on cold start and open loop when hot as well.

later
Jeremy

 

Did you put any fuel additives in it?

I put a container of Gumout fuel system cleaner additive in my tank once - I'll never do it again.
The car ran like complete crap (just about what you're experiencing - engine miss that couldn't be pinned down to any one cylinder, poor performance, etc.). I started replacing many things - new distributor rotor, cap, spark plugs, spark plug wires, air filters. Nothing huge, but for my budget - a bit of money spent. Nothing fixed it. I ran the tank down to empty, filled up (no more additive left, obviously) and hot damn, the car reverted to how it was before the additive was added - which was running good. So, what had been my situation is as I'd replace things, it would seem to be worse - because my tank was less and less full, which meant the engine used more and more additive.

 

Like I said originally, this was an ongoing problem over a couple mths now.

Getting worse instead of better with no signs of it being internal engine.

I think I night burn it, isnt that what insurance is for? lol

 

What about the ECM?

 

Dataloggin wise and emulating everything is hunky dory.

Anything I've tested likewise such as solenoids, grounds, injectors is good as well on the ecm side.

 

lol

Im convinced my car just hates me.

It absolutely hated the other distrib I picked up lol Runs better with my old one from last december. So back it went and my old one back on.

No timing lite, but I scribed the base and intake and also marked the rotor location. For giggles I dropped it in the proper location with my eyers closed cause my friend bet me $5, was worth a laugh, got to keep the humor up for projects like this cluster.

However now it gets interesting. Yesterday a on/off moaning noise started. Couldnt figure it out. Today I was down there fooling around with it again(making me miss football and it will get burned) and had a oxygen sensor to swap in for grins since nothing conventional show up, whatever I can swap out free or cheap has gotten the nod. Anyhow new Bosch(very cheap with discount yes I know to run Delco but none to be had easy here without ordering) doesnt thread in cause Bosch is crap :-) Put the old one back in.

Start car with vaccuum gauge on it and pull injector leads, they all drop about 1 inch hg per cylinder. So again everything is being affected. Moan starts again. I pull the tubes off the AIR pump diverter valve and its blowing like mad hatter while idling(did have 02 unplugged as well to rule out as many things as possible) so I unplugged it and got the valve to close.

Idle is better, not perfect and the miss is far less apparent, but still there somewhat. Idle has actually dropped below 600rpm to about 575.

Car runs/drives/feels better, but mileage is still horrid, did a loop of 610 around Houston and still burned prolly a qrtr tank.

Some progress, not there yet but getting closer.

 

ok so obviously its something in the electrical system causing this...
what new distributor did you put on there?

put a new cap, rotor, ignition module and coil on the old distributor, replace all the wires and spark plugs... make sure you set them to the correct gap... and see if that doesnt completely cure the misfire. sure itll cost you like $200... but that way you can completely rule out any of the ignition

 

U missed where all that has been done already

 

It still hates me.

Today the gas gauge is back to 5/8 tank meaning I used a 1/8 or a little over, which puts the mileage closer to normal. Love the accuracy lol

It is doing better with the AIR line pulled from the exhaust manifolds..............

Stupid car.

I got school and some stuff to do later, so I will see how it goes

 

what about your charcoal canister, try taking the gas cap off and running it, as well as plugging the main line into the side of the throttle body to eliminate it, it definately won't help you right now anyways so it's not wasted work if it's something else.

I had similar problems, although to be honest i didn't chase them down the way you have, what fixed it was doing a complete reset, meaning i went thru the whole process of setting TPS, base timing, and all that, start to finish the whole thing so as to get the IAC tps and base timing all dead on to each other. I know it sounds basic and stupid, but after all the hairbrained things i THOUGHT it was, all it turned out to be was poor calibration.

 

Yeah, I have pulled the charcoal canoster out of the sytem to, its not it.

My TPS is good if its under 1v its a speed density setup.

Timing was dead on until I moved it on Sat and I know its almost dead on now because I marked it and scribed the intake/distrib :-)

Im going to go thru everything and reset the IAC and timing when I can get my hands on a timing light.

The car is a hell of alot better right now, and I got a few errands to do t-nite so we will see who it goes.

 

IF you haven't checked compression to rule out a valve not sealing, you haven't checked enough.

 

If a valve wasnt seating it wouldnt pull 19.5 inches of vaccuum however. Its about a 100-1 shot at that point.

I'm not at home, Im at school without my own tools and such to work, so everything is add hoc on what I have to use and when I get to use it.

Also it would be traceable to a single cylinder which so far has been a useless task.

Fitting in working time around school and work isnt fun either. Lack of tools and diagnostic tools blows lol

Its still doing the same crap, less so, but still there.

Pulling the valve covers and compression test are coming up when I can bum a compression tester and have some more daylite time to work on it.

It has me kind of pissed, due to time and $$$$ constraints I may just eat it and take it to the dealer. Cant cost me more than the gas Im wasting.

 

One valve not sealing completely would not ruin the whole engine's vacuum. It would make the vacuum have a periodic "tick" downward then back up. Never know...

 

i had a similar problem or maybe the same one, and like you part of it was bad plug wires which i know you mentioned you already changed, but did you check for carbon tracking? and also i had a secondary ignition misfire. this is coming from the diagnostic sheet from the dealer so maybe you know what it is since im assuming your in some kind of auto school. but i just chose to get screwed over and let them fix it and now it runs great. hope you figure it out.
kevin

 

I am tending to agree with the guys that are stating its ignition related. 9 out of the 10 times i have a miss its the secondary ignition system.

Theres this one little spark plug on my TPI that always fouls up and causes a nasty miss and kills my fuel mileage. I clean/change that one plug and it runs great.

I hope you charging system is putting in enough power to fire those spark plugs.

Good luck!

 

I'm back home now.

Car is still for crap

Compression is 180-185 on all cylinders after 4-5 cranks. Go to 7-8 cranks and its 210-215 on all holes

Leakdown shows nothing on all cylinders. All withing a couple percent and all under the 10% mark.

Still cant trace it to any one cylinder.....

Around town mileage sucks horribly still, can feel/hear the miss. Amazingly coming home from school it pulled 22.5mpg with the car loaded and doing 75-78 mph steady. Did 1100 miles in under 16 hours. Of course once u get to 22-2400 on the tach under load the miss is a lot less prominent.

Nothing shows wrong using my MT2500 Snap On scanner or running DataMaster either. So would seem to be located on the pass bank cyl's 2-4-6-8, least it appears to be, but unable to confirm it in any way shape or form.

Can feel it on cold running and when its warm.

Did catch it puffing from the pcv valve grommet exactly once. Car was pretty cold and had thick oil in it. Havent seen it before or since, doesnt jive with compression or leakdown numbers either.

Looks like I will be pulling the intake off nice and slow and inspecting as I go.

Might have a hookup using someones old Sun Scope stuck in a corner at a shop, got to make some phone calls on that.

Or I go back to my original plan and burn it.

later
Jeremy

 

Why don't you just bring it by my shop and I'll setup a PROM to see if that makes a difference.

Are you still running the stock injectors?

Have you tried replacing the Memcal and or ECM?

Are you back at school or back down here in Houston?

Let me know,

Mike

 

Already swapped proms/memcals/ecm's/custom tune/stock tune/fill in the blank.

I'm back home anyhow, no offense but I like it here lots better than there :-)

Still showing nothing.

One of the guys in a town over has been around for awhile and helped me with weird stuff and lent me tools back in my formative years. He has an old Sun Scope, so I'm heading there at 8am and seeing if I can figure something out by looking at the firing traces on all the cylinders.

After that. its getting torn down with a microscope cause I'm all out of ideas.

Although burning it and using the money to get something with an LS1 and a 6 speed is appealing :-)

later
Jeremy

 

Ran it on a sun Scope to see the ignition traces. Guess what I found?

Nothing :-) Yet again something else that checks ok, I'm so lucky.

So I ended up tearing the intake off and found it was leaking on the egr passage, was all wet with oil, etc.

Who knows maybe it will clean up after that.

Wasted my time part of yesterday and today resealing the intake and guess what? I wasted my time again.

Yeah me.

Burn it or LS1 T56 it.

 

What brand plug wires, spark plugs, distributor cap and TPS sensor are you using?

On my TPI, when the TPS sensor went out I went to Advance auto parts and bought a GP Sorensen part. Not to knock that brand or anything, but my TPI just would not run correctly with that part. It literally ran like crap and set a trouble code. I took it back and they gave me a new one, i ran back and forth about 4 times.

Finally the counter guy said, im sorry, i cant give you another one, i am giving you a refund. I took the money, went down the street to Oreillys and bought a Borg Warner TPS and it solved the problem.

Also, the GP Sorensen wires are complete crap, they stop conducting after a couple months.

Bosch platinum spark plugs tend to accumulate heavy carbon deposits that are nonconductive and creates a loss of power and a miss when you rev it at low RPM'S that you can actually hear. Higher RPM's it kind of dies away. I have encountered this in both Camaros... a Carb LG4 and my LB9. Replacement of the fouled plug or plugs results in a fine running vehicle.

The other thing you might want to check is for a functioning EGR valve, if it is opening at part throttle it will create a bog and kill your fuel mileage. Disconnecting the line and plugging the valve with something like a screw, and a subsequent test drive might yield a better running engine.

Just try to keep in mind that a replaced part may not mean its a good or quality part. Bosch 02 sensors have a history of being incompatible with the Chevy system, (in my experience so has GP Sorensen)

 

You do realize you have to calibrate a new TPS using a volt meter, and that you can't just plug it in and expect no trouble codes. You need to set the range properly.

It may not have been the sensor at all that was the problem........

 

Yep, adjusted each one to .54 volts using a voltmeter. Yes i adjusted each one to specifications. I also made sure i had a good connection each time. It would run, but it would act just like a bad TPS, but transmit a voltage to the ECM. When i would hook up the scanner, it would show a changing voltage.

 

I just read this thread with interest, as I have had similar misfire issues for 2+ years. I was ready to roll the car off a cliff. Have you tried watching the engine running in pitch dark? If not, make sure you wait to let your eyes adjust, and then look at everything very carefully.

I eventually fixed two ignition issues, but since they were related to my intake swap, I don't think they would apply to you. But I'll throw this out FWIW.

The first problem is detailed here:

https://www.thirdgen.org/forums/diy-...le#post2780790

I had the EXACT same problem (strange blue light show in the dark) that was also fixed by moving the neg. battery cable.

The other problem was my coil wasn't grounded properly. When I switched intakes, the instructions said to move the coil from the intake to the rear of the cylinder head. I did, since the coil wouldn't bolt up to the new intake. Current was jumping from the coil itself to the coil's square "ring" since it wasn't grounded properly. So I made a bracket, put it back on the intake and problem solved.

Again, might not apply to you but I throw it out there anyway.

Good Luck.

 

Sorry I am kind of new here, I noticed a few people talking about Dataloggers and scanners, I have previous experience with the A-pexi S-AFC and the MMCD datalogger for the Eagle Talon, I really wish I could get a datalogger for my 91 Trans Am, sorry to jack your thread, Mods can delete this post if need be, but just saw a few people referring to loggers. Thanks.

 

if its on only one side of the engine, it sounds to me like an airflow restriction in the exhaust. i understand that you checked and replaced the cats, but what about the rest of the exhaust? running duals? check that side. if single w/ y pipe, check the even side of the pipe, ya know. just throwin it out there, id hate to see you waste all of your time and money trying to fix something, when it was simple. its happened to me before, and it sux. always remember the basics