cmd296stcl

Sorry to bother ya'll with another diagnosing question, but i'm stumped, and so is the local garage. I have a '91 z28 with 30k original miles. I took it in into the garage because it had an unsteady idle - it would drop 200-300 rpm like it wanted to stall, then would catch itself, and ad go back to the set idle point. Also, under hard acceleration, she would stutter like she wanted to choke out, but never did. Before having taken the car to the shop, she had never stalled. The garage gaver her a tune up, but when they gave my car back, it was worse than before - it could not accelerate past 20 mph. I immedieately took it back, but after the garage had the car for 4 weeks, they were unable to correct the problem. They put new air/fuel filters, new fuel pump, new injectors, new distributor, new plugs/wires... but nothing corrected the problem. Now, my car stalls out once it is warmed up (runs better cold than when it is warm), the exhaust is much quiter, the car is doggish, and the idle drops ~200 rpm's when put into gear. Sometimes at very low speeds, like when pulling into a parking spot, the car surges forward (almost like it momentarily got some of its power back). I'm running out of ideas, and i don't want to take it to another garage until i have some idea what the problem is. I would appreciate any suggestions ya'll have. Thanks.

Sonix

iTrader: (1)

I won't even pretend to understand the complexities of the computer controlled fuel injected versions of the 3rd gens (shudder).

My first thought is; what codes are being stored in the ECM?
I would bet large sums of money on it being a sensor or ECM component, and not a hard part (like a banana in the tailpipe, or a crushed catalytic convertor, or a wiped out cam...)
But like I say, I don't pretend to understand the electrical bits, so someone else here can help you more with what sensors to test with your voltmeter, etc.

Gramps

iTrader: (1)

does that shop have a diagnostic machine or have experience with f-bodies? if im having a problem w/ my maro and i dont know the problem, ill either take it to the dealership and have the pros look at the diagnostic machine or ill take it to a guy that owned 3 other camaros and ran into the same problems i did.

cmd296stcl

Sonix, my first thought was something in the tail pipe, but i crawled down there with a flashlight and looked up both tailpipes - I didn't see anything. I'll keep the cat in mind, thanks.

cc 82Z-28

iTrader: (7)

If it is the original convertor, I would be suspicious of it since I have experienced first hand what a plugged convertor can do. I believe you can have the exhaust flow measured to be sure, but with lower cost of new convertors as well as the high flow units out there, I would replace it anyway if it is the original. As for the computer stuff, well I'm not much good there. Good luck.

sofakingdom

iTrader: (1)

What's the fuel pressure while it's doing all this?

I don't mean, what's the fuel pressure sitting in the driveway idling; I mean, while it's in the act of lurching, stumbling, surging, and unable to accelerate past 20.

formulascott

First start with the map sensor, is it pluged in/ and working the only other thing I think could be the problem is that the ECM could be going bad, they will not always trip a check engin light, also check the O2 sensor. And like said before take it to a dealor or somebody with a OBD1 or better scan tool.

Sonix

iTrader: (1)

these cars are far too simple to need a "scanning machine", ODB1 requires the fine art of bending a paperclip to nab those codes.
So, first order of business, DO YOU HAVE A CHECK ENGINE LIGHT? That's pretty important there.

oh no, I said I doubt it was something like a plugged exhaust, i'd put my money on an electrical gizmo before a mechanical failure (especially at 30k miles!).

DENN_SHAH

iTrader: (1)

codes are a good place to start, they can give you a starting point, but they don't always tell the whole story & don't always get set & stored.

knowing what the fuel pressure is during the event is needed to rule it out or confirm its the problem. depending on what im looking for, its anywhere from #2 to around #8 on my list of things to check.

a movie of the data stream during the event can tell a lot about whats going on.
if you have a laptop & can get a cable to connect to the ALDL, the cables are pretty cheap, you can make your own movie. you can download TunerPro and the data stream file you need & get better info than what the store
bought scanners give.

as a test to see if the exhaust is the problem, you can remove the O2 sensor & give it a run down the street & see if it makes a difference.

cpullma2

iTrader: (4)

Would deff check codes if there are any. Might even pull throttle body off and give it a clean. I had something like you had, and had to clean TB, and put a new egr valve on. Took me forever to figure this out (no codes). And my car only had 38,000 on it when this happened.

cmd296stcl

Ok, let me see if i can answer everyone's questions and maybe help narrow things down:

Formulascott: A new ECM was swapped in, made no difference.

Sofakingdom: The garage said the fuel pressure was "fine". Last I set it, it was at 48 psi.

cc 82z - 28: The cat is original i believe, but the rest of the exhaust is aftermarket.

Cpullma2: they check engine light has never come on. I thought my EGR valve was bad a few months ago, checked it, and there was a bad vaccuum line to it, so i replaced that. At that time, the 'check engine' light was coming on. It is not at this time.


Thanks so much for the ideas guys, keep them coming!

johnnyboy

iTrader: (3)

sounds like maybe bad timing, did they adjust the timing at all?maybe they loosened the distributor when changing the ignition components? which would cause your car to act like this if base timing wasn't set and the timing was changed your car would run differently when it is cold and when it is hot.

iac valve could be part of weird idle

cmd296stcl

idle control valve was also replaced

johnnyboy

iTrader: (3)

did they check the timing?

cmd296stcl

not that I know of - but that would explain the difference in drivability once the vehicle has warmed up?

chevymec

iTrader: (5)

Maybe they tried to set the timing and didnt unplug the dizzy wire to set it.

johnnyboy

iTrader: (3)

yeah, if the timing wasn't set right or if it was adjusted unknowingly that would explain it the difference in drivability once the car has warmed up

Gramps

iTrader: (1)

wow checking codes only gives you a baseline, it could be some very simple problem which is having a cascade effect on the rest of the systems. if you go and it says the MAF sensor is flashing, that means it could be 4 things; defective MAF, defective relay, defective ECU, short in wiring. thats over 500 buks to fix that. if you take it to a dealership, they hook about 30 wires to the car and run it, then tell you everything. itll prob cost 50 buks (a 20 min job) and they could come back to you and say "your fuel filter is clogged so at WOT the engine is starved and the MAF sensor regulates air/fuel ratios and the comp flashed that code.

sofakingdom

iTrader: (1)

And that was, as I described, WHILE IT WAS MALFUNCTIONING; and not simply while sitting in the driveway idling, right??

You need to know what the pressure is WHILE IT'S SCREWING UP. That means, while driving down the road, trying to get on the freeway or wherever else it shows its problems. NOT just sitting in the driveway.

Should be 38 psi or so during a steady cruise, with no real changes or fluctuations in the gauge indication; should rise to 45 or so at WOT.

D's89IROCZ

There was no MAF in 1991 . I am leaning towards Coolant temp sensor . Did you pull the codes yet ? Need to get them ( if ) any .

cmd296stcl

No i have not pulled the codes myself yet. I guess I should go make a purchase so I can do that. Thanks again for another idea.

How can I check if it is the coolant temp sensor?

sofakingdom

iTrader: (1)

Easiest way is to just swap it out. It's cheap.

Then when that doesn't do anything for it, check your fuel pressure WHILE IT'S SCREWING UP; i.e. while driving down the road trying to accelerate.

D's89IROCZ

you don't need to purchase anything. Just look in the tech articles here and you will see the way to do it. All you need is a paper clip and maybe
a notepad to record all stored codes. If you have problems with it ...toss it up here and we can help ya out


good luck man!

sofakingdom

iTrader: (1)

If your Check Engine light isn't coming on, then there's no codes.

If there's no codes, then the paper clip method of reading codes is worthless, because there will be no codes to read.

You could conceivably take it to somebody that has a scan tool, maybe even somebody that does it for free such as VatoZone, and see what the scanner reports that the ECM is being told that the coolant temp is.

On the other hand, if the ECM doesn't see the coolant temp reach a certain reasonable threshold within a certain reasonable amount of time, then it will set a code, and light the Check Engine light. But if the light's not on, then there's no code, which means the ECM is seeing the engine temp vary in the pattern it is programmed to expect as normal.

For help to be helpful, it must be accurate. That's been a problem on these boards alot lately.

Don't fixate on one thing somebody tells you, just because it's what you want to hear; and ignore common sense, because it seems unpleasant for whatever reason.

Check your fuel pressure, WHILE IT'S ACTUALLY MALFUNCTIONING. That does not include "it was OK before it started doing it", "sitting in the driveway idling", "mechanic 'said' it was 'OK'", etc.

See my signature.

jayk2k3

iTrader: (2)

maybe the timing chain jumped a tooth?

johnnyboy

iTrader: (3)

check your timing!!

icemanohio69

i would check the ignition module and the coil i think just about any parts store can check it just a thought good luck

Sonix

iTrader: (1)

man, you've got a schwack of suggestions here, and they're all over the map.

It's got a new fuel pump, and new fuel filter, that's the only reason I wasn't gravitating towards it being a fueling problem. But I think sofakingdom is trying to get a message across here, but i'm not quite sure what it is...?

Might as well check it, at least to shut 'em up. I've been trying to prove him wrong for a while now, so far, no luck. My car runs fine, partially from this help though, so I can't complain.
Grab a fuel pressure gauge, you can rent them from the parts stores (free). The type to use on an FI car, not a carbed car obviously. Run the line into the passenger seat, and get buddy to read it while you drive around. Get him to glue his eyes on the gauge and watch it. When you feel the car do something "funny", shout "now!" and he can tell you what it's doing.

I'm always leery of garages that give up on a car. They shouldn't have a license to practice automotive mechanics if they can't fix a 16 year old FI car.

Timing is also super easy to check, worth eyeballing. Disco' the 4 pin connector and reset timing to ~8* or whatever. Borrow/beg/steal a timing light, shouldn't take more than 4 minutes to do.

DENN_SHAH

iTrader: (1)

i have seen enough new fuel pumps that were bad to learn to always check them, just like sofa probably has. there may be a problem with the fuel pump wiring or the relay.

there is also a possibility the outer ring on balancer has slipped.

the shop he went to may be like a lot of dealerships are, there are a few dealers here if you come in with a 94 model car, they tell you its too old for them to touch, to take it else where.
it surprises me at the number of young techs that haven't had any training in the older systems.
i have given up on a few cars here & there, mainly because the owners didn't want to spend the money needed to repair them.
but there is one red 93 buick skylark 3.3 that i did say " i don't know & i have no other ideals" on.
i must say it was a strange feeling doing it.
a no start with firing injectors & no spark, no problems with the wiring, a correct new balancer, a new Delco crank sensor, 3 different new Delco modules & 1 aftermarket module were tried. i even asked a few thousand other mechanics, it always came back to the module.

cmd296stcl

Thanks for all the suggestions guys - you've given me plenty of things to look at; but like Sonix said, the suggestions are all over. Perhaps if i try to better describe my problem, maybe ya'll can help me narrow it down to a few things. First of all, I wrote in my initial post that the car would not accelarate beyond 20 mph, but after taking my car back to the garage, that problem was corrected. I don't know how, but it was. Also, the car idles poorly when both warm and cold (fluctuates 100 rpms or so), but ususally only stalls when the car is warm. (build up of back pressure - clogged cat?) Also, my exhaust is much quiter now. I don't know if this is because the car is just making less hp or not, but it may be another clue to the puzzle. In addition, my car drops about 300 rpms when put into gear as opposed to being in park or neutral. Any ideas with that? Finally, gas mileage has dropped too. Numerous members have asked if i have checked the codes, which I haven't. No check engine engine is light is on either.

Still sound like timing and/or fuel problem to you guys? hope this helps you guys come up with some more ideas. Thanks again for all your help.

sofakingdom

iTrader: (1)

No check engine light = no codes

Put "codes" out of your mind.

It's not timing. "Timing" is one of the "universal scapegoats" that people who don't understand what's going on, especially newbies to the field, will try to blame everything on. The other super-popular one is "EGR".

"Timing", in the sense of it being something you can adjust, is a function of the location at which the distributor is bolted into the engine. That doesn't fluctuate, correct itself, change symptoms depending on vehicle speed and conditions (that is, it doesn't change from correct to broken and back, or whatever). In other words, the symptoms you describe CANNOT POSSIBLY be caused by "timing".

Put "timing" out of your mind.

It's most likely fuel delivery. Sometimes it gets enough fuel, sometimes it doesn't. The reason the exhaust "seems quiet" is because instead of being a full-time engine, it's part-time just an air compressor, and nothing more.

Until you verify that your fuel pressure is correct UNDER ACTUAL OPERATING CONDITIONS and WHILE IT'S SCREWING UP, all the posts on the Internet on all message boards put together, won't be enough to convince the car to run right. Go check your fuel pressure as described. Duct-tape the gauge to your windshield and go drive around.

johnnyboy

iTrader: (3)

if the timing is not set properly the engine will react differently at different temperatures, i had my timing set advanced and it ran fine warm but when it was cold the engine would stumble and sometimes even stall, so i retarded the timing and now it runs perfectly at any temperature.

f1ls1

iTrader: (2)

Check the ground strap at the rear passenger side head. They may have broken it when they changed the dist cap and wires.

Shadygrady

iTrader: (3)

Had asimilar sounding problem with my 305 before i did the engine swap.Car would barely go then jump forward harshly and jerk.Rpms were erattic.Almost caused a wreck.Turned out to be two stripped wires touching from my coil.It was in my ignition system were i fuond my problem,but it took three hours of traceing wires.I had no codes and fuel pressure was fine all the time.The wires were apparently pulled to hard or something when the 305 was rebuit because they were stretched really bad and frayed.Like I said,just a thought.Good luck,hate it when my women dont act right!

Zap Racing

Having a TPI, and having dealt with a problem very similar, before U do anything else, U need to remove the throttle body plate--clean it--remove and or check for correct operation of the Idle Air Control motor (or replace) and reset the TBS --throttle body sensor. I'm not going to even attempt to explain how to do it here. Do a search like I did for these subjects and u will find more info than U need dealing with this--it has been covered about a gazillion times before--several excellent articles --step by step--by Vader I think--, but more than likely the biggest problem is the TBS--if it is set incorrectly, it is telling the computer one thing and the comuter is trying to respond to that information.

D's89IROCZ

Could have a burnt bulb in it ....so why not at the very least check it . Doesn;t cost nothing .....so why not ...then ya know absolutely .


Also for timeing .... he could have a stripped bolt hole in the manifold ...allowing the timeing to change , because the hold down bracket isn't tight enough . Sure it's unlikely but it could happen

Thats why we are all here just to say stuff to check .... I see your point though if someone was saying ...go buy this ....go buy that . But just checking stuff can't hurt nothing .


Now onto more diagnosis y'all .

bilms01

I you are sure the fuel pressure and ignition system is in good shape. try looking for a small vacuum leak somewhere since you had an idle problem when you took it in the first time. Most garages don't do anything more than replace the plugs and maybe a couple of filters if you ask for a "tune up"

cmd296stcl

Thanks again for all the ideas. Keep them coming!

84_86_92cam Z28

im not shour the easyiest way to check(only if a manual fuel pump)the cam lobe. the other thing i would check is if the rotor is in the right location.O and checkif the wires are touching or arching .The easy way to see if the wires are arching start it in the dark look under the hood for arching. Also see if the torque converter is seezed? just some kinda other ideas.

Sonix

iTrader: (1)

If the car lights up the check engine light when you start it up, the bulb isn't burned out, and if it doesn't light up when it's running - no codes. If you never see the check engine light light up (at startup), then it's burned out.

Have you tried any of these suggestions yet? ?

No fuel pump lobe on his cam, '91 car.

cmd296stcl

Sonix, I actually have payed any attention at start up. But I know it was working about 3 months ago when i had the egr changed. (ended up being a hole in the vacuum line).

I have not gotten a chance to try any of the suggestions yet. I'll be out of town until tomorrow night, but starting Sunday i'm going to be testing a few things. If I can't get it by wednesday, it's going to ARD in Houston on thursday/friday of this week. Either way, i'll be sure to inform everyone what the problem is. Thanks again for everyone's help.

johnnyboy

iTrader: (3)

just to clarify things it's a tps-- throttle position sensor

Slayre

For a quick check on the cat, put your hand over the exhaust when its running. If it sucks your hand its possible it needs replaced, also the smell usually like sulfur when they go bad.

cmd296stcl

Slayre - Thank you

Zap Racing

OOps--sorry--thee memory isn't what it used to be!

See what all U guys have to look forward to in the coming Years--LOL!

robsgta

The hand over the exhaust trick isnt the propper way to check a catylitic converter. Some people get scammed into believing that there cats are bad at shady tail pipe shops with that method.

As for the sulfur smell all fuels contain some type of sulfur compound in them some more than others. If the smell bothers you try another brand of fuel the smell means nothing.

The correct way to test to see if your cat is bad is to buy or rent a catalytic back pressure gauge, you just plug it into the the port where your 02 sensor is and at idle it should read under 3 1/2 psi and under a full throttle load the pressure will rise momentarily then drop between 0 to 2 psi as the air rushes through the cat. If the pressure raises to 5,10, 15, 20+ psi you have some type of obstruction or melted/damaged cat.(you can make one if you get a 0 to 20 psi air gauge and plum it to a 3/8 hose with the same thread on it as your 02 sensor to do this test)

Personally I think its your fuel pressure all because the pump was replaced doesnt mean its the right one or a quality pump. Theres a difference between buying a 50 dollar cheap pump or an a/c delco 200 dollar pump or even higher after market racing pumps.
I have replaced several pumps on different cars that have all your symptoms.
Place a fuel gauge on your fuel rail turn your key and see if your pressure is stable (constant) or does it drop ?
Your pressure should raise 8 to 10 psi under WOT/load not drop.

Rob

Slayre

robsgta, it was only meant as a quick check providing he didn't have the tools or parts and as far as the smell I was always told it would emit a strong odor if it was bad I'm no expert on cats, Ive only experienced the hand trick one time with a friend who was having his replaced. There was a difference before and after. I would recommend checking it the proper way as robsgta has mentioned before you ever decide to get a new one.

robsgta

Hey CMD296 what part of town do you live I might be able to stop buy and check your car out.
I'm on the north side...

Rob