I've been working on this for about 2 weeks now and I'm really getting tired of this.... It's been to the dealer more than it's been on the road! Here's what I originally started with, and still have the same problem now....

The car is a '93 corvette with the LT-1 engine & automatic. 178,000 miles. In February, the car would not start one morning. Took it to the dealer, and they replaced the optispark, and while they were at it, replaced the water pump (was leaking on the opti). Flushed the cooling system & put it back together. It ran fine until May. Then, again, it wouldn't start one morning. Took it to the dealer, and they replaced the ignition coil and plug wires. Ran fine for two months, and the same thing happened again! Wouldn't start one morning. Took it to the dealer again, they performed all the electrical checks, and said they found nothing. They thought maybe it was the timing chain. I pulled it from the dealer and sat in in the garage...And there it sat until now....

The symptom: Car will start, and BARELY idle. You give it gas, and it sputters, backfires, and dies. When it dies, it seems like it's kind of dieseling and fading away.

I decided to do my own work from here on out....I bought the factory service manuals and have been following the process of elimination...Here's what I've checked/replaced:

Started with the least likely, but to be cautious, I changed the timing chain. The timing did NOT jump time, but the cahin was sloppy..Needed to be done anyway..

There are NO codes being thrown by the computer. The computer does generate the code 12 when it goes into diagnostic mode (does that mean the computer is ok?)

Checked the following sensors: MAP, IAT, ECT, and TPS all were in spec (min & max span).
The EGR was not stuck open at idle.
I checked Vacuum and it's at 10" Hg, which is low, but I don't know if that's a cause or a symptom. I pinched off all the vacuum hoses one by one and vacuum did not improve.
I unplugged the connectors on the AIR pump, and that did nothing.
I checked fuel pressure: 43 PSI Key on, 39 PSI running. Does not drop off when key off.
Checked the IAC valve, looked dirty, replaced, did nothing.
Replaced the PCV
Took a spark plug, connected to plug wire, started engine, the spark looked a little weak. Bought a new coil, and ICM module. Made no difference.

The O2 sensors were replaced < 2000 miles ago.

Turned off the lights in the garage, ran car & looked for spark jumping and saw nothing.

When I replaced the timing chain, I put a timing mark on the engine and the balancer at #1 TDC. Connected a timing light to #1 and the distributor seems to be firing consistently at the same spot, maybe 20 deg advanced. Took the light to the rest of the wires, and they all were firing.

The only thing I have left to do logically is replace the opti but, I don't want to play the parts swapping game. Is there ANY way to diagnose if it's working correctly without special tools?
What ELSE can I try first...I've exausted the GM manual....

Any signs that the PCM might be good or bad?


HEEEEELLLLLLPPPPPP!

 

Sounds to me like its time for another opti swap. My buddies 93 has done exactly as yours, he has put 3 opti's on it in the last 5 months. Good Luck
Scott

 

Just curious because I didn't read it up there...but have you checked ALL your spark plugs and wires, too? Also, I noticed your fuel pressure looks good, so the fuel pump probably isn't bad, but have you checked your fuel injectors, too?

 

Checked all the plugs/wires.....What should I check on the injectors if the pressure holds when the key is off?

 

That's exactly what happens when you take off a vaccum line. I say vaccum leak. Hows the rubber grommet that the PCV valve goes in?? or the brake booster grommet?? Basically, anything that seals up the intake. I imagine even the common rear intake leak if you have blown all the rtv out of it. What was your MAP reading at idle?? I would think that if your vacuum is that low at idle the car should die if you touched the throttle at all. I would replace the rubber grommet under the MAP sensor, they are the worst about leaking if you have ever pulled the sensor.

I never heard of a code 12. Here is a list of OBD-1 codes from ShoeBox's website.

http://csce.uark.edu/~jgbertr/rb/Dtcs.htm

Jeff D.

 

The "12" is what is displayed 3 times when you jumper the disgnostic pin to ground, then it should flash the actual code after that if there are any stored....

Additional things I noted:

Took valve covers off, all lifters are functioning properly...

If I disconnect a large vacuum line, the car actually runs better....Still backfires & sputters with gas, but Idle improves....But I don't know if that means that with reduced vacuum, the timing retards and somehow compensates for the root cause problem???

I hooked a vacuum pump to the Map sensor, plugged the hole and the more vacuum I apply, the worse the car runs....Does that mean anything?

ttt

I need help!!!

 

Ok, just to break it down. (Makes it easier for me anyway )

Motor only need three things to run.
1. Air -vaccum, intake sensors, filter, etc
2. Fuel -pressure, injectors, filter, etc.
3. Fire -ignition coil, distributor, wires, etc.

So first you got to figure out what one it is. In the first post the dealer was looking at "fire" both times you took it to get fixed. You said that it ran fine after each time, though for a short period.
It looks like you have gone through every major sensor expect the TPS and the O2 sensors. How many miles on the O2 sensors btw? I hear that they only last about 100k miles max. If one of those goes bad the computer cann't adjust anything correctly because that its only feedback on engine operation.

Are 93 Vette LT1's Speed Density or do they use a MAF??
If they use a MAF then:
I would first try disconnecting the MAF sensor plug. This may set a code but you can clear it if it does later. TEST: If the MAF goes out (by you unplugging it) the computer will stay in open loop and start using the volumetric effiency tables that are in the computer. It will not take info from the O2 sensors either while in open loop. Basically, I would see if the car runs better in this "limp" mode because it disables the computer from using a possible bad sensors. If no change is noticed then we can look somewhere else. Just starting with the easy and FREE stuff first.

On a differnet note, have you done a compression check all the way around? You may have two or more that are way low having that many miles. This can cause the diesel effect if one relies on the other to turn the crank. If so you can try closing the gap on the spark plugs by 0.05-0.10 and see if that helps idle any.

Jeff D.

 

Poorman, thanks for reminding me...

Both O2 sensors have been replaced < 2000 miles ago.....

The TPS sensor was checked and it is ok on both low and high end....

The '93 is a speed - density system.

 

When you say you disconnect a large vacuum line and it runs better, that makes me think you are getting too much fuel. Look and sniff the vacuum hose to the fuel pressure regulator for presence of fuel.

 

I checked for fuel in the vacuum line and it was ok....Fuel pressure readings looked good too....

I also hooked a manual vacuum source to the pressure regulator & it worked as expected, tried dropping down the pressure (38 psi running), and it made no difference....

Should I waste the money on an opticrap today, or are there other things I should try????

 

Does anybody have anymore suggestions? I'm really getting tired of this now...I'm thinking about pulling that chevy 350 and getting a nice reliable pontiac 400 for the vette!

PLEEEEEEEEEASE help!!!!!!!!!!


TTT

 

Your vacuum looks pretty low.I am guessing intake leak or possibly the brake booster.Check all vacuum devices.