Alright, here's a puzzle for you good troubleshooters...

My vette is a 91 convertible 6spd. Everything is stock except some flowmaster mufflers, k&n drop in filter, and a hypertech chip (unsure of details, done by previous owner).

My problem started off as a "shutoff" problem. When I would be under full throttle (tested to be gear independent, but usually after some rpms had climbed in 5th/6th after a few moments of being at WOT), I would just lose power. The engine would idle (tested by pressing clutch) but no power in gear. The problem would resolve itself in 5-20 seconds or by me turning off and recranking the car in motion (clutch pressed). During the problem, when I had the clutch pressed, watching it idle, if I pressed the gas, it would start to get some rpms, then die all out. I never lost electrical power to the dash or got an engine code.

Eventually, it started the same type of behavior (only wants to idle) after was is warm for some period of time. This could be either riding down the highway for 20ish miles or if I warm it up in the garage, shut it down, and fire it up 20-60 minutes later. It would run "OK" cold, but felt weak. Twice, it has actually gotten so bad I couldn't get it to start.

Now, it idles and drives pretty terrible hot or cold. The gas is a few months old at this point, which is probably not helping. Plus, she hasn't been driven any due to this problem (already had it towed twice).

I have replaced: fuel pump, fuel filter, egr valve (it tested bad at the diaphram), ignition coil, dist cap and rotor, ignition module and capacitor, plugs, plug wires, and o2 sensor.

I drove it for a while with a pressure gage on the fuel rail, and never saw any problems there (after replacing the pump, did not check it before). I have checked the driver's side exhaust backpressure at the air injection port per the book, and it was good (passenger side is obstructed at this port). I have also checked grounds, specifically the one behind the passenger seat (fuel pump grounds here, I think), the trans bell housing, and the main engine ground. I took the seat one and main block grounds off and cleaned them.

I suspected it was running rich due to the old plugs being black and some occasional white smoke when I was testing (just after having the battery disconnected). I have checked out everything on the "running rich" condition in the book (code 45 I think) except for injectors and fuel in oil. I have also gone through the troubleshooting charts for the ignition system and fuel delivery system with no problems found.

I'm about to pull my hair out over this one. Any ideas?

SteelBlue91

 

Not sure what you mean by DTC 45, 'you think'. Can you clarify?

Do you have loping at idle?

What were the numbers for fuel pressure? Static? Idle with vacuum off of FPR? Idle with vacuum ON FPR? Did you check pressure bleed-down time, which will tell MUCH!

Post back on that, then we'll go on diggin' deeper...

 

Code 45 is the code for "running rich" as detected by the o2 sensor. The diag page in the book for this code gives all of the items to check for this situation. While I didn't actually have the code itself, I used that page as a list of things to investigate.

Idle is not too terrible cold, but does have some surging when it is hot. When it is hot, it would almost die out, there would be some rattling sound (detonation of some kind? sorry, I'm not a mechanic), then it would regain normal idle speed, then repeat. Total cycle time of 5ish seconds.

Fuel pressure before startup (after pump primed) was 42-43. Idle was 35-36 if I recall correctly. I had it out on the road and revved it up several times, and it would show back up in the low 40s. All of the numbers agreed with the book's spec, which I believe says something in the low 40 range before crank, 6-10 less idling. Sorry I don't remember the specific values, I did the fuel checks over a month ago.

I did not check fuel pressure with vaccum off of the regulator. I did check for a faulty diaphragm in the regulator (one of the checks for running rich). I pulled a 14in/Hg vaccum (top end of "typical values" from the book) on it many times consecutively, and it held with no problem. There was no gas leakage into my vaccum tube.

I did not have noticable bleed down in the fuel pressure that I could tell. In fact, I would do some cold checks, warm the car up, then come back an hour later and it would have 40+ still showing (some rise from heat buildup, I am guessing). Of course, I don't know what is considered "good" for bleed down time, but I figured if there was a problem, I would have seen it then.

Unfortunately, before I saw your reply, I got fed up and sent it to the shop. I've been fighting this thing since January, and seeing all of the other vettes around in the nice weather was the last straw. Of course, it is not guaranteed that they will find the problem; I am still looking through the book for ideas in case they don't.

SteelBlue91

 

Good job doin' the diagnostics!

Loping idle is highly likely a vacuum leak.

 

Good idea with the matches, I will have to try that.

I did spray around with a can of throttle body cleaner, but was never able to get any response. I will locate all of the lines and target them specifically with some starter fluid (more volatile).

Thanks!

Steelblue91

 

Well, long story short, it looks like the problem was the prom chip in the ECM. I talked to them mid-day and they were confident that it was the ECM, but they could only change the box and use my old chip. Apparently, even dealers cannot get replacement prom chips. I called back later on and they said that the box change "did the trick".

So I show up and post bail and the guy says he'll go get it and drive it around. After a while, nothing. Guy shows back up and says they had trouble starting it, and that the tech was checking it out. After about 15 mins, the PR guy comes and gets me. The tech is there with my vette; hands me the hypertech chip and says that I can throw that in the garbage. He apparently had a used chip that he put in for me. He said he couldn't charge me for it since it was used, but it should work fine.

The ride home went pretty well. Between the ancient gas (1 part from early Jan, 2 parts from mid-late Feb) and the different chip, it is a whole different car. Hopefully, things will be better when I get the old gas flushed out and put in some fresh (and get the tires rounded out). I will also be shopping for a backup prom since I have no history on the one I am using now.

Any reccomendations on a fuel system cleaner additive for cleaning carbon buildup? When I pulled my old plugs, they were pretty black, and the tech mentioned that my new plugs (changed about 2 weeks ago) were black already.

Thanks again for the advice, cwb; I'm sure it will come in handy in the near future.

Edit: Tried to crank it about an hour after I got home, no go. Looks like they have a problem back on their hands at the shop.

SteelBlue91

 

Don't go to the shop - I think they don't have a clue. I'll get this before they do - if we can do each test properly. And you're gonna' NEED an FSM, or an online subscription to Mitchell's online, or ALLData, and a multimeter.

CALL, DON'T GO, to the shop, and see if they'll supply you with printouts from these online resources as needed. If they can't, then they have nothing for you (and I can't imagine how they've drawn any conclusions thus far).

Check for codes first. How long was the ride from the shop? Did the car fully warm up? If not, you'll have no codes. If so, get the codes, THEN call the shop, and see if they'll print out diags for those codes, for FREE. If not, get the FSM.

If no codes, and it warmed up fully, post back...

 

It is already in the shop, but I will likely pull it soon. I had some words with the a-hole working on it. He claims my new plugs were gapped at 0.050, which is BS. They come out of the box at .045, I gapped to .035, end of story. He also tried to accuse me of changing the chip back to my original hypertech, which is also BS.

My main reason for sending it back is that I have technically paid for them to fix the problem, which they didn't do. I should hear back soon, or I will just cut my losses and get out.

I live about 18 miles from the shop, it is downtown, and I am more rural. It was high traffic hour when I drove home, so I know it got plenty hot before I ever even really moved. I actually drove it back in, but it was pretty close to dying when I got there. After I shut it down in their bay, it would not recrank.

No codes at any point.

I already have a factory service manual, 1st thing I bought.

SteelBlue91

 

Have you tried testing the MAF? Might need a scan tool to look at the grams per second flow. Another test is to tap on the sensor to see if you have any engine stutter. Next would be to do an injector balance test to see if you have an injector or 2 leaking or spraying to much fuel, then try an injector clean service. Also, look at coolant temp sensor readings along MAT readings to see if they are out of range. A lazy coolant sensor will show a cold engine when engine is warmed up, thus having a fatter fuel mixture, same with the manifold air temp.

 

MAF reading is a good idea with a K&N.

Too much filter oil will foul the filament in the MAF, and you'll never know it otherwise (but the computer sure will). Get some MAF cleaner, and clean it CAREFULLY. The filament is fragile.