Ok, here we go. A friend is having an annoying problem with his stock '91 convertible. Last summer his car started dying on him sporadically. At that time it was showing a code for an EGR. So he has it replaced and drives it home ( 15 miles) with no problems. Well yesterday God smiled on us and it was in the 60's so he decides to drive it. Drives to town and around all night without an problems. On the way home, (4 hours later) he felt the urge so he wants to blow her out a bit. As soon as he does this, she starts sputtering and trying to die. Well he makes it to a service station and thinks maybe its out of gas and the gauge wasn't showing. Not the problem, only hold 7 gallons. He tries to start it and it takes 3 tries and when it finally does, it blows black smoke out the exhoust and dies. Tries again and it start right up. Drives it home (10 more miles) at around 40-45 mph and doesn't have a sinlge problem. Now hes saying hes going to sells it if we can't figure out the problem. Anybody out there got a clue? :confused: :banghead:

 

The '91 is designed to tell you what faults are occuring. Pull the DTCs and you might have an answer.

 

Sorry for my ignornace but what is DTC and how do you check them? Are they checked the same way the earlier C4's?


[Modified by ZAD54, 3:16 PM 2/3/2003]

 

See http://www.c4vettes.com/ecm.htm for instructions.

 

Diagnostic Trouble Codes. Was the '91 the 1st year for the LCD digital speedo readout ('91 - 96 I thought)? If so, the info on the C4vettes.com page should work to pull codes.

I agree: get it to spit out the DTC, to find out what's going on.

 

I guess I forgot to mention that the car is not throwing any codes now when it is having its problem. I talked to him again today, and he said it acted like someone just shut off the key and then turned it back on while he is driving. I'm leaning toward an ignition problem or ecm since it is getting plenty of gas which is evident by the black on him white bumper. :banghead:

Lone Ranger: I believe the '90 was the first year for the new dash and new difital speedo in the center of the dash pod.

 

Try the positive post on the battery. Is it dirty, clean it up really good and use dielectric greese on it. :)

 

I had similar symtoms when my MAF sensor was on its' last leg

 

Its a '91 so its a speed density system, but thanks for the try. Maybe its the map sensor. :confused:

 

My 88 had similar problems. The annoying thing about it was that it would run fine for a while, then all the sudden stall, sputter and die for apparently no reason. It was initially mis-diagnosed as a bad EGR.

Turned out to be a defective oxygen sensor that was making the car run rich. The rich mixture, in turn, plugged up the catalytic converters. When the cats were cool, the car would run OK (although down on power), but they'd eventually plug up and stall the engine. Sometimes after a few miles, other times after 20-30 miles.

The diagnosis for this problem is to run the car until it's hot and begins to stall. Take off the AIR check valve from top of one of the exhaust manifolds, and connect a 0-15 psi pressure gauge (most vacuum gauges also have pressure-measuring function). Take reading at ide and at 2,000 rpm (I think), and compare pressure readings to specs in the service manual. (And yes, the procedure for this is outlined step-by-step in the service manual.) Remember, this has to be done when the cats are hot enough to plug up.

In my case, I didn't need to compare to specs because the gauge shot up beyond scale as soon as I blipped the throttle. All 3 cats were fried . . . even after I replaced the oxygen sensor, they kept plugging up.

Hope this helps.

 

OK, I just talked to him again and he is going to change the O2 sensor. He also told me that the problems started last summer right after he put some injector cleaner and Costal (?) fuel treatment in the tank. Do you think the additives could have messed up the O2? Sounds like a good explanation to me. :confused:

 

TTT

 

Yep, I think you found it.

 

The oxygen sensor is a wear item, so it can't hurt to replace it.

But if he had been running rich since last summer, then the cats are probably fried already. In the case, the problem will continue even after replacing the oxygen sensors. So, he should test the cats anyway as described in the service manual (Helms) . . . it's free and takes only a few minutes.

As a general rule, always TEST first, THEN replace. The "guess-and-replace" method of diagnosis can get out of hand and very expensive.

If he needs to replace the cats, a cheaper way to go is to replace the pre-cats with straight pipes, and install an aftermarket main cat. It'll still be smog-legal in most states, and will make the stock exhaust sound better.

(And yes, I've always suspected that in-tank additives are harmful to oxygen sensors, but I'm not a chemist or an engineer, so I can't say for certain.)

 

The car was driven no more than 25 miles after the problem began last summer. It was driven to his parents garage and then to a mechanic for the EGR replcement. After the repair, it was drive straight to his garage. The car only has 42,xxx miles on it and is rarely driven so I don't think it ran rich for very long unless a faulty EGR can cause a rich condition. Anyway, he said he would replace the O2 anyway since they were cheap and I have about got him talked into changing the pre-cats for the non-cat head pipes. And if I can squeeze a few more dollars out of him, maybe a Random cat for the main.