John Currie

I have a 91 Z28 with a 305 TPI. For some reason above 2500 rpm the car acts like it has a governor on it - full throttle, no power. I have replaced the plugs, plug wires, distruibutor assy, Ox sensor. The chevy place said that the timing is being retarded and giving a code 43 but that the knock sensor is good. This unit has the ESC incorperated into the ECM. I disconnected the knock sensor wire to make it think that the sensor was good all the time. Still runs bad. It runs great for the first minute and a half then it falls apart. A friend said its's a clogged cat in the exhaust. But I had them removed years ago. It has SLP headers and Flow Master ex. Besides that it runs great for a short time. They said it could be the ECM or the EPROM. I am so tired of replacing parts that are good. I'm going broke. Can somebody help me - please??
jcurrie@ajaxtocco.com

Vader

If you simply disconnected the detonation sensor, the EST likely just retarded the timing ot base timing as a result. You need to install a resistor to "fool" the ECM into thinking there is an active detonation sensor installed.

92blue

iTrader: (1)

Don't start just changing parts because someone suggested something. For some reason, the ecm is pulling your timing back. The code 43 is evidence of that. What you need to figure out know is why thats happening. What do your spark plugs look like? Have you checked them yet?

What chevy place are you referring to? A dealership? How did this problem start? Suddenly, or was it something gradual?

The prom contains the esc circuitry, not the ecm. However, it is rather rare for that to go bad. Don't suspect the ecm at this point either, its more than likely not the problem.

Either your knock sensor is sending false knock signals, or your engine has significant detonation problems. Wiring a 3.8k ohm resistor in line to trick the ecm will keep the timing from getting pulled back. however, if your engine really is detonating, you need to have the timing pulled back to avoid engine damage.

John Currie

Thank ya'll for the replies. The Chevy place is the dealership up here. I have replaced all the plugs. They didn't seem to look bad. This sort of symptom used to happen way up in the rpm scale when the car was going about 95 mph. Then one night my wife and I went out to dinner and by the time I got back it was running really bad. I took it to a local shop and they could find nothing wrong. They changed all the gas out and recirculated some kind of fuel system cleaner. The car really has never run right since that night. That's when about almost a years worth of changing things began. The car runs great when first started - about 1 1/2 mins. Then bad on acceleration. If I let it gradually build it seems to be ok (very gradually). If I throw the car in nuetral and rev it up it does fine - but when I load it with the road it stumbles all over itself. This 3.8k resistor - is this in series with the knock sensor or in parallel with it to ground?

92blue

iTrader: (1)

I do not recommend you do the resistor though. If you have detonation problems, then all you will do is end up causing engine damage if you wire that resistor in line. Has the knock sensor been replaced at any point? If the wrong one was put in, it could cause what you are saying.

Plugs didn't look lean, or have missing pieces when you took them out?

When you rev it in neutral, are you stropped or moving?

If you are moving, then I would think its safe to say that you have detonation problems, and not false knock.

If you are not moving, it doesn't help any.

The ecm won't pull timing back when the car isn't moving.

John Currie

The knock sensor has never been replaced. The plugs looked normal when I replaced them about 3 months ago. I just went out and pulled one to look at it. It does not have much wear - there is a slight tan color begining on the end of the insulator where the center electrode comes out. There is a very slight, light power white color on the base of the other bent metal piece that connects to the threads. When I rev it in neutral it revs fine whether the car is moving or sitting still. You say that means detonation but it does this when the knock sensor is not connected at all also???

I connected a voltmeter to the knock sensor and drove it down the road. When I turn the key on it goes to 12V. As I start the car it drops to about 4V until the car is started, then it goes back up to 12V. After about 15-30 sec it drops to around 1V stays there for a little while then goes back to 12V. It stays at 12V through out the RPM range and at all speeds. I shut it off to see if it would go back to 1V during that short time after it is started. It did it again, but then went back up to 12V. The car smelled like fuel when I got back. It has smelled that way alot. What is wrong with this car?