Hey everyone, got a problem, trying to figure out what the cause would be.

1994, lt1 automatic,

when i first get going, and give it gas, it always chokes, sputters, bangs, etc. until today i would completely let off the gas when it did that, then just be reeeeaaaalll easy with it for a few seconds, then it would seem fine. today, i decided to keep in the gas to see what it would do. so it started doing it, and i kept giving the gas, and it just kept doing it even worse, even seemed like it was just about to die, so i let up, a TINY TINY BIT, (still giving it enough that it should be accelerating pretty good), and then POW, it instantly started running like champ and took off, like brand new no problems ever.

it almost seems like it was just all plugged up and stuff and blew itself clean or somethin, but some of the noises almost sound like metal banging around. it wasnt loud or anything, and i could just be exaggerating because im worried about it, but it NEVER has any other problems. EVER. has 160K+ miles on it. i do have to change the intake and valve cover gaskets, it has a brand new transmission, and it never does this if i have been driving it for any more then 5 minutes and try to accelerate. would this just be a clogged cat? it has the corsa exhaust system on it. bought it last may, but went on deployment, so i've really only had the car for 1 1/2 months, but no other problems to indicate anything. ALWAYS starts up like a champ, always runs strong except for this. any ideas? thanks. oh yeah, one more thing, ill clean the exhaust tips, and then the inside ones on each side will be dirty/black the very next time i drive it.

 

I bet this occurs between 100 and 140 degrees coolant temp, after a cold start. Does it do it right away when hot (ie. above 180 or so) but running for less than a minute or two?

Drive it while watching the digital temp. readout. Report back.
A buddy has a 94 that does this. It is in storage, we've chased this one for a little bit. I still got a few tricks up my sleeve for that car come spring.

 

ok will do. i can tell you now tho it does it for sure below 180. as far as above that on a first start tho, i dont know, i will have to try to do it.

 

If it sat for more than a month or so while you were deployed, BTW thank you for your service to the country, I would drive it carefully to burn off the gas in the tank then fill it with 93 octane and a new fuel filter before trouble shootng anything. E10 has a shelf life of around a month maybe two then you are on your own.
Dave

 

then I finally cleaned the MAF and things got a lot better.

 

ok just got back from driving it, on they way out of the neighbor hood though, i had to sit and wait for almost 10 minutes before i could go, because of stupid amounts of traffic coming out of the church, and none of em would let me in. it got up to 149* before i could go, and when i got on the gas, it ran like a champ. this is the only time on the first run of the day that it hasnt choked and sputtered.. etc etc. but i usually dont have to sit and wait to get on the road. then on the way back, an hour later i started it at 155*, but by the time i got on the road it was 181*, and again, it ran like a champ. actually, it ran awesome today. i will have to see what it does when i go to work in the morning. I usually only drive it maybe once a week, but even driving it every day since thursday when the other car went in the shop, its still been doing this on the first drive every day(except today)


thanks. it actually sat for 6 months, i filled it up with 93 and stabil, but since ive been back its had 3 fill ups, always with 93, and i put fuel system cleaner in it on the first fill up, super concentrated. i havent changed the filter tho, didnt even think about that.. i will have to pick one up.

did it only do it on the first run of the day though?

 

My gut feeling is the water temp sensor or wiring. The only thing you changed is the engine temp. If it is reading very high compared to actual engine temp it will be painfully lean and do many of the things you describe. If you have the C68 heat / AC module it is possble to get the engine temp the ECM or PCM or or or the box that runs the engine some seem to very **** about ECU ECM PCM designation.. sigh.. If you have C68 and no FSM I can at least give you the sequence to find this info on a 91.. Once warm the error is trimmed by the O2 sensor and might explain your problem.
Dave

 

Great, your car plays games with you like mine does. When I bring home the tools I need, it plays nice, when I don't it decides to run like crap. They have a mind of their own, I swear it.

Intermittent problems stink on ice. I'll tell you right now, DO NOT just start throwing parts at it. Best thing try to do is catch it in the act.

FYI, to the best of my knowledge the LT1 has two coolant temp. sensors one for the ECM and digital display, and one for the analog gauge.

I would find someone with a decent scan tool as it would be a huge help. I cannot over emphasize this enough.

 

well, this problem seems to be more complicated then i thought. let me see if i understand you right, you're saying that if the ECM is reading the temperature as hotter then the actual temp, its gonna cut the fuel? if that was the case though, why would it all of the sudden be perfectly fine and run perfect? damn these cars. took me almost a year to get familiar with all the funny things it did/does, i have NO idea about the things with a Corvette yet.

yeah, i wasnt planning on just throwing parts at it. fuel filter is just a good idea. after all the car has 160k miles on it, dont know if or when it was changed last. i have actually been trying to find some decent corvette shops around here, the only two i could find out about, one was out of business, and the other guy is an *** clown. do these cars have an obdII port? i have a somewhat decent scan tool that can record some info, im sure getting it to do the problem wont be a problem..

 

94 and 95 are weird birds. They have the OBDII 16 pin connector but run ODB1 software. Unless you have a OBDI compatible tool with the ODBII cable, your out of luck.

Yes, if you have no idea if that filter has been changed it is a good idea to do so. Do a search here the fuel filters are a little tricky but once you know the tricks, not a big deal.

To answer your question to ddahlgren's statement.

Electrical issues can be on/off and intermittent very quickly. This sounds electrical in nature, but not saying it is.
If the problem is occurring when the car is "warm" and just going into closed loop then it can be a on/off like problem. My buddies 94 is like this, I can tell you exactly when it WILL do it.

Like I said, in the spring, I got some things to try on that car.

Looking at data and monitoring fuel pressure while driving are going to be your starting points.

 

hmm.. well i will have to check if its obd1 compatible. i dont remember.

i havent had much experience with electrical problems, but something is telling me that this isnt electrical. but since i dont know a whole lot about these cars yet, im gonna have to go with other peoples experience.

it did it again today this morning, when it was cold outside, car was at about 85 when i got going, did its usual, and as soon as it stopped i looked at the temp.. sure enough, EXACTLY 140. man.

I dont think my scanner moniters fuel pressure and all that, i just now that when it reads a code it shows some other important readings with it, and i can tell it to read those at any point with another menu choice, but i dont remember ever seeing fuel pressure. what if the fuel pressure is constant the whole time, what would that mean?

 

Your car is doing EXACTLY what my buddies does. The car goes into closed loop operation which means instead of relying on sensor inputs and internal ECM settings for fuel control as in open loop. The ECM uses 02 sensor feedback for fuel trim corrections starting at 140. His car like your runs perfect in closed loop.

The popping and such I think is a lean misfire which backfires thru the intake.

I never had a fuel pressure gauge on his car, I WILL this spring.
There is NO fuel pressure readout on the scan tool. You must use a mechanical gauge. Only newer cars and many Fords have a fuel rail pressure sensor that allows for viewing on a scan tool.

I say it is electrical in nature because any kind of little bad input to the ECM during open loop operation could be causing a lean condition. I was not able to see any odd readings on my scan tool while the problem occurred.

I cleaned the MAF no change, but worth doing. Voltages and frequency of the readings from the MAF in range.

I never found any vacuum leaks on his car.

I tired disconnecting the air injection system, no change.

I think I tried unplugging the MAF and this left the car in open loop all the time (and ignoring the O2's) and it ran fine on speed density. Bad part of this is the fans will run all the time too...Something worth trying and see if it runs better, at least once.

It is not a 02 problem, since the issue occurs in open loop, in closed loop the car runs perfect using 02 feedback.

FYI, I had the Opti-Spark off his car and the cap off, no issues there, resealed and put back in. Still did the same thing, I had to replace the water pump gaskets, leaking, so I checked on the Opti, while in there.

If you put your scan tool on it, see what your coolant temperature sensor reads before you start the car and see what the intake air temp. is. They should be close.

ddalgren's post got me thinking, as I recall I checked the coolant temp. reading versus my I/R Thermometer gun aimed at the T-stat area, got about the same reading. This is something else to check.

Hopefully, this makes some sense.

 

yeah, makes sense. i will unplug the maf tonight and see what it does in the morning.

i left my scan tool in the car thats in the shop, and they close before i get off work, so i might not be able to do any of that till this weekend.

i was reading something about a common problem from opti spark failing is misfires... so you could be right about that.

as for that coolant temp/intake temp, when it was colder yesterday morning, the car's reading said 52, and my thermometer said 50(not the thermometer on the climate controls, i have my own), granted thats no scan tool reading, but at least they were close.

so what im am understanding is that if i unplugg the maf and it runs perfectly fine, that means its going into a closed loop, and so i would need to figure out WHY its doing that to fix this problem, right? would that be the ECM going bad?

next time i have any time to do anything i was planning on changing my plugs and wires, ill just add fuel filter and check out the opti while i do that.. hopefully soon.

 

I don't think it is the Opti. Doesn't make sense. If spark was the issue or the Opti, why would it run ok cold and above 140?

If you unplug the MAF and it runs better all that tell you is that it runs better in OPEN loop and without the MAF.

Like I said it seems the issue pops up from about 100 to 140 degrees coolant temp. before it goes into closed loop. Once above 140 and in CLOSED loop as you noted it runs great.

I think my buddies 94 and yours by the sound of it are running lean during this time. Why, I'm not sure on his car yet. Definitely not pulling the trigger on any parts yet until I get more testing done. I WILL say this, AT LEAST ON HIS 94, I believe it is a fuel issue or a fuel control issue...ie. sensors, ECM,etc...

 

right, it runs great once it goes into a closed loop. sorry, read your last post wrong. sorry if i keep making you repeat yourself, these cars are just so damn different then any other car ive had. hell, i can fix my other car with my eyes closed, and know exaclty what is causing any particular problem, with the Vette im still pretty unfamiliar with it.

well, i will have to see if i can find someone that has a gauge i can used for monitoring the fuel info. i will post back tomorrow with how it did while the maf is unplugged.

ok, open loop is where the problem is, when it is getting its settings from computer settings etc, instead of just the 02s which are good since it runs fine in closed loop. so if it does run fine with the maf unplugged, this means either: (correct me if I am wrong ) 1. that the maf is effed and its reading/sending the wrong information to the computer or 2. the computer is interpretting the info wrong or calculating wrong or controlling wrong, etc. again, sorry, just trying to make sense and do some troubleshooting at the same time. So other then unpluggingfy a maf, is there any kind of tool or any other way to test if it is working right? Computers are a whole other story...

Just curious, how much would a new ECM run me? It would be awesome if its the same price as a new computer for my GTP, only 200 bucks!!

 

If you tink it is open vs. closed loop just unplug the O2 sensor or sensors..

 

well, i tried out your suggestion on the way to work this morning, unplugged the maf, ran perfect. no problems at all...

 

Wow, it IS just like my buddies car. Ok, so lets recap.

Once into closed loop above 140 degrees coolant temp. the car runs perfect.

If we unplug the MAF and put the car into open loop it runs perfect at all temperatures.

Now, that being said, could the MAF be giving a false reading?
Yes, but you'll need to monitor it via scan tool to verify and know what your looking at. It will give you it in voltage and in Grams/second.

Fuel pressure could still be an issue, in closed loop via the operational 02 sensors the ECM corrects this lean issue with increase pulse width and the running issue is not present.

You are correct in your understanding, for the most part.

All inputs need to be correct for the ECM to operate correctly. Trash in=trash out. That being said, ODB1 computers do not have the "intelligence" of OBDII, they do not have the built in logic circuitry.

Could there be a ECM issue, it is possible but I would verify ALL inputs to ECM are good, fuel pressure is good all the time, etc...

Some sensors are priority sensors like the MAF and ECT. The knock sensor is not a priority sensor in the ECM's mind. When you take out the MAF, the MAP, IAT, ECT become the priority sensors as it has to run in speed density mode versus mass airflow mode.

Without actually testing your car, or my buddies in the spring, I can only go by what I know so far and as far as I am concerned I believe they are running lean at these points, backfiring thru the intake, via a lean misfire.

My reasoning why it is NOT a ignition issue is as follows:
-No issues during cold start up and at low coolant temps.
-No issues during closed loop operation.
-As soon as closed loop is reached problem instantly goes away with 02 control of fuel.
This last reason is enough to put the ignition to rest in my mind. On my buddies 94, it could be making noises and back firing thru the intake with OL (open loop) indicated on the scan tool. I could watch it go to CL (closed loop) on scan tool and the backfiring would INSTANTLY stop. 02 sensor feedback is for FUEL control NOT ignition.

To add another wrench to the monkey works, his car like yours, is a 94 and has a flashable EPROM. So anyone with the right software could have got a "tune" in your ECM. God only knows what previous owners did.

I'm not sure what a ECM would cost you???

What I would do:
Verify all inputs are correct to ECM. Check MAF for proper voltage-grams per second.
Verify fuel pressure at all operating conditions. Idle, WOT, etc...
Check injector ohm resistance should be around 12ohms.
You may even have to get the injectors flow tested??
Try tapping on the MAF with a screw driver, NOT TOO HARD, see if the engine changes at all at idle. Clean if you have not already done so. I did on my buddies 94, no change.

Read this carefully, hope it makes some sense. I don't mind repeating myself as it sounds like you are learning and trying.

 

Worst comes to worst, I would give Gordon Killebrew a call before I spend money on parts. That guy knows C4's better than his own hand. He wrote the darn service manuals. Knows wiring diagrams, wire colors, circuit numbers, all from memory. Smart guy.

If you don't have a Factory Service Manual get one!!! Only the Red ones, the white ones are only preliminary ones.

 

The LT1 uses a few sensors when the car is cold for calculations:

MAP (manifold absolute pressure)
MAF (mass air flow)
TPS (throttle position)
IATS (intake air temp sensor)
CTS (coolant temp sensor) car has two. one on water pump is for ECM, one in head is for gauge

Go nuts.