Ok.. my car is a 91 z28 convertible with the 305 lb9 tpi engine with a 5spd manual trans.

my car has been sitting in the garage with this problem since mid october

The car starts and idles almost perfectly, but won't rev above around 2800 rpm (sometimes it won't go above 2400, and sometimes it will go up to 3300). If you press the gas down to try to go above that rpm range, it stumbles and then dies if you don't let off the gas.

I pulled the following 2 codes from the ecm

22 - Low voltage at tps or fuel cutoff relay open or shorted to ground.
34 - Low voltage, high vacuum, at map sensor

I checked the tps and the voltage did jump around a little to much so I replaced it. The new one is smooth as can be. This didn't help any.

I replaced the map sensor (the readings on the old one are fine though) and that didn't do anything either (which was expected).

I replaced the ECM for the hell of it, which of course didn't help any at all..

I replaced over 10 feet of vacuum hosing which led to a slight increase in rpm.

Fuel pressure test revealed 41 psi, and didn't drop much at all up to around 2800 rpm. Haven't tested volume yet though

Replaced the fuel filter

When I spray ether into the throttle body while working the gas with my hand, it has no problem at all reving up to redline.

My father checked a couple of the injector wires on an osciliscope or whatever it is and said they looked great.

I'm thinking it almost has to be something in the fuel system.. I definetly have spark since it works with the ether, and it don't miss or anything with the ether..

any help would be greatly appriciated.!

I realy need to get it back on the road.. My 88 chevy truck just broke it's pathetic 700R4 for the 4th time.. so i'm stuck in a 90 v6 tbird and it's PURPLE

 

Dan,

You father's idea of checking the injector pulses with the oscope was good to determine if the injector pulses were present. If it was a digital storage scope, did he save the traces? If so, if there are cursors on the scope, you (he) can measure the pulse width to the injector. Compared to the "off" time of the injector, the duty cycle of the injector pulses can be calculated. If the duty cycle was around 80% or better at WOT, the ECM and wiring are O.K. (I'd be a little surprised if they weren't.) If it's not a digital scope, you can still guesstimate the duty cycle and see it it's close.

Since you've covered just about everything else, I'd guess the injectors are not flowing correctly. The only way to solve this is to remove and service the injectors.

Check with Cruzin' Performance for details on the procedures.

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Later,
Vader
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"I cannot take this any more... Saying everything I've said before..."
Adobe Acrobat Reader 4.0
KaleCo Auto Parts

 

i'm thinking about just replacing the injectors while i'm at it.. anybody have any sugestions for new ones? Right now my car has no mods at all except for K&N filters, but I plan to supercharge it sometime in the future. I'm only lookin for 350 hp out of it with the supercharger, and a complete exhaust system. So i'll probably end up running realy low boost.. just want to keep up with the newer ones

 

Maybe your cat is plugged up? Sounds like the symptoms...

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West Coast GM Shootout 2001!
1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.25 @ 107.18 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

 

nah... she seems fine.. just saw her chasin a mouse around the garage...

oh.. that cat.. i thought about that awhile ago, and the engine revs fine when I spray ether into it. That suggest it's a fuel problem.. plus the exhaust is flowing out prety good too..

 

92Z1LE posted a message in new thread about his car basicaly running rough and dying if the gas wasn't kept down some.. my car did this same thing for a while right before it started doing what it is now. Anybody think this might be related somehow?

 

Actually, mine is sort of the opposite. If I rev it a few times it usually fixes the problem. So far higher RPMs aren't an issue.

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92 1LE 25th Anniversary Heritage Package
305 5-speed
Hooker Headers, SLP exhuast, K&N filters

 

when my car first started doing this, i could just rev it some and it would go away.. but most of the time i'd just let it sit and wait... and talk to all the prety gals

 

anybody have any suggestions at all? i prety much tried everything and nothing seems to fix it the thing that gets me is it works fine when ether is sprayed into it.. so it almost has to be a fuel problem.. yet the pressure is good, put a new fuel filter in, and the injectors all seem to be working fine.. hell.. i've practicaly replaced everything on the engine so far hmm.. maybe if I change my air freshner hanging from my rear view.....

 

Are you saying the engine will rev through the entire rpm range on ether? My guess would be a weak fuel pump or bad coil. With a weak pump the car might not have adequate pressure beyond a certain rpm, and with a bad coil it wouldn't have enough voltage. Is it timed correctly? Do you have access to an autoxray? I'm sure you'll find the answer eventually on this board. Wait for more responses than mine of course - since a coil is $50 and a fuel pump is a lot more. If you have access to a known good coil that would work you could use it to rule that possibility out.

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89 Iroc-Z LB9 TPI auto 2.73 posi:
3" Dynomax muffler, K&N filters, TB bypass, 3" pipe in place of cat, cleaned and flow-matched injectors, accel 300+ wires, cap&rotor, synthetic oil, flexlite transmission cooler, afpr, !air pump&diverter valve, kicker sound system, soon to attempt engine swap...

 

Dan,

I ROCK may have it - but here's the gig......

I am WAAAY out of my element on this - I just do not have enough diagnostics experience on TPI cars to tell you with confidence that it probably is a fuel pump or coil.

I do know this: I have witnessed two separate instances with TPI cars that suffered the same symptoms as yours, and in one case it was the fuel pump. In the other case it was the pickup in the distributor.

You already have a major investment in $$$ and time, so it's probably time to call in a pro.....or even better.....got any buddys close by that drive 3rd gen cars ?

Swapping parts to test only costs you time and sometimes gaskets, whereas buying parts get expensive quickly.

This is why I ROCK's diagnosis makes sense:
(other than the fact that we've seen this before)

The fuel pump may work well enough to supply rail pressure up to 2,800 RPM, but that doesn't mean it's functioning properly at 4,000. Have you used a FP gauge and swept thru the idle-4,000 range at WOT ?

If the pump is starving the system, it will show up in a road test with your buddy riding shotgun and reading the gauge (sneak it up under the hood so it just appears at the bottom of the windshield).

The pickup is cheap....I'd just replace it.

Please stay with it and keep us informed, because this is obviously a real-world problem, and that is the major reason for this board - maybe some other guys will hop in here.....

Guys that have had exactly the same symptoms and figured the solution out. Good luck.

BOR

 

I wish I knew some people out here with some.. but all my friends either have 4th gen ones or mustangs..

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91 Z28 Convertible
99% stock.. K&N Filters http://www.marks-mall.com/91z/1991.htm



[This message has been edited by Dan91Z (edited January 19, 2001).]

 

ok.. i decided to re-check the fuel pressure, but could not get ahold of my friend to borrow his gauge.. so I decided to buy one from autozone.. and it turns out the pressure drops from 43psi to below 30 when I start the car.. on his gauge when i checked it last time it only dropped about 3 psi, but not that I think of it, the needle seemed to be shakey.. Seems like his gauge was broken or sticking or something..

just think how much money I would have saved if I would have bought a gauge when this first happened...

Oh well.. at least i'm getting somewhere now!

i'm replacing the fuel pressure regulator tonight, and hopefuly that fixes it..

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My `91 Z28 Convertible

 

Dan,
where in KY are you at. we have the same cars, and if you are close to Louisville, we can get together and swap stuff. No one else around me has 3rd gen so I know where you are coming from. Good luck, I went through the same problem, it was my fuel pump.

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92 Z conv
5-spd B&M shifter
home-made ram air, air foil, headers, no cat, TB bypass, kevlar pads, slotted rotors, BMR panhard, alston SFC, 3.42's
when it warms up Edelbrock STB maybe springs

 

Dan, mine used to do that kinda randomly though. I'd be driven down the highway at 55 or so, and the engine would cut out. Wouldn't rev past about 1500 though. Shut it down and let it sit for a fe mins and it would be fine. Everything seemed fine until I did a fuel pressure check. Lost it all in less than 15 mins. Sent the injectors out to cruzin performance, he confirmed, my injectors flowed like $#it. Needed to be run in the cleaner several times...now all I gotta do is replace the plug wires I recently burnt on the new headers, and replace the fuel pressure regulator which somehow died while my injectors were out for servicing...
Corry



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Corry Lazarowitz
clazarow@voicenet.com
clazarowitz@hotmail.com
SLP 1 3/4" headers, 3" cat back (stock cat )MSD ignition. MSD Blaster SS coil. More soon when funds are avail...

 

Sounds sorta what my car did when the battery was dead as a doornail... or course i couldn't start it either...

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1991 Z28 305 1LE, TES headers, 3 Chamber flowmaster, Hi-reving cam... the usual street racer

 

thanks jmon92conv for the offer.. but i think i have it figured out now..

at least i learned not to trust a 15+ year old fuel pressure gauge

 

Dan,

The new fuel pressure information changes the diagnosis a bit (I was working on the assumption that the pressure was really 41 PSI as you originally stated). Since the pump will maintain 43 PSI when the ignition is on, and the pressure falls to nearly 30 when you start, I would suspect the pump.

You stated that the filter is new, so that possibility is out. It could also be the regulator, but the fact that it will maintain good pressure with the engine off indicates the regulator is not sticking open. Does the running pressure change if the vacuum hose is removed from the regulator? If the pump is bad, the pressure will not change much if at all. The normal "drop" is about 6-8PSI.

You can performa a flow test on the pump by removing the pressure line, put the line into a container that is at least a half gallon, and operating the pump for 15 seconds. The pump should be able to flow at least 12.5 ounces in that amount of time - bare minimum to feed injectors at 44 PSIG at maximum flow rates. The long-honored standard is one pint in 15 seconds, and a new pump should flow three times that easily. If your pump is just barely at one pint, it's probably failing under pressure. If the flow is at least a pint, go back to the injectors.

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Later,
Vader
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"I'm gonna talk about some freaky sheet now..."
Adobe Acrobat Reader 4.0
KaleCo Auto Parts

 

ya.. seeing the fuel pressure drop that much on a new gauge realy sheds light on the matter..

As soon as i found that out, i unplugged the vacuum hose from the regulator and the car ran up to 5k, but would still cut off.. so i plugged it off and it seemed to work prety good.. still haven't done the volume test yet, but i'm prety sure it's the regulator..



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My `91 Z28 Convertible

 

the new regulator is finaly on.. and the old one was in prety bad condition.. there were some prety big tears on it..

The car will hold steady up to around 3700 rpm now which is almost a 1000 rpm improvement.. so replacing the fuel pump is definetly next.

I do have 1 problem with the regulator though.. after installing the new regulator, i only get 30 psi of pressure. it shoots straight up to 30 when i turn the the key to the on position and stops there.. so it looks like i somehow ended up with the wrong regulator.. although the part number is right.

i'm going to replace the pump (flowed 12 ounces in 36 seconds), then put another regulator in..

I did notice the new regulator was about 1 MM taller than the old one, but thought it was from wear on the old one and didn't think much of it.. any thoughts on this? I seem to always get faulty stuff

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My `91 Z28 Convertible

[This message has been edited by Dan91Z (edited January 23, 2001).]

 

I have a 92Z28 convertible, blue with a black top. Anyways I had a problem simlar to yours and it turned out the my inition module was shot. If I remember correctly I may have had the same codes even.
For your fuel pump, try this, use the pressure gague hose off you schrader valve on your fuel rail but dont put the gague on, put the hose into a gas can. Then use a jumper wire from the + battery terminal to the + on the fuel pump relay connector. You could then run your fuel pump and you wouldent have to remove your fuel lines or even start your car. Just my .02

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Convertible, L98, Mini-ram, ZZ9 cam, Comp Cams 1.52/1.6 steel roller rockers, Trick Flow 23* heads, K&N's with chopped airboxes, March underdive pulleys, Edelbrock ceramic coated TES, Flowmaster cat-back exhaust, Dynomax high flow converter, MAC subframe connectors, Accel box & coil, 3.23 rear gears, white face gagues, and NOS!
When I get back from Kosovo I will be installing these on-order items!

 

there is usually a set screw on the top of the regulator, that will raise or lower the pressure when turned