my buddy's 96 silverado doesn't have cats, so his check engine light is always on.... the other day he's going about 70 down a highway and pulls up to a stoplight. when the truck stops and drops to idle, the whole truck started shaking and tried to die kinda, like it was bogging down at idle. he turned it off, popped the hood to check for obvious smells or noises and whatnot, and everything seemed normal.

he started the truck back, and the check engine light wasn't on anymore, but it was still running fine. he's just wondering why it would do that?

btw it hasn't done it since, but the check engine light is still not on.

when the light was on, the truck would idle at about 1500 for about a minute or two... now when it cranks it just idles at 700 from the start... not sure if that's related just thought i'd throw that in.

any ideas?

 

Most be somthing with the ecm

 

well since it only happened once and hasn't since i kinda figured it was some kind of brain fart in the computer...

 

What did he do to the ECM to override the faulty 02 sensor readings? Anything? I think even the 96 engines need to be "told" that there is no cat there so no engine light will be thrown. If the computer setting are similar to mine on my 99 then any number of things can go wrong if you just remove the cats. COT temp readings, AFR readings on both banks, etc.

Plus is the cats aren't there and if he doesn't have enough back pressure then the engine will run lean because there won't be enough fuel at idle for it to run propperly. Not only will the sensors read faulty signals form the 02s but with little backpressure the cylinders more more freely and will "push" air threw the motor faster without having enough fuel to compensate.

If it is running lean then, if you leave the truck go like this it can burn up the rings too.

 

Yeah but if the engine is flowing more freely and allowing more air to go out quicker, it's also allowing more air to come in quicker and more freely so it should balance out. He could always take the plugs out and see if it looks like the thing is leaning out.

 

Its not really as much "More" air as "faster than it can handle without modifying something else". I fixed my rough idle on my 91' by closing the gap slightly on the plugs. Tell him to try that for starters. Either that or higher octane fuel.

 

aight i'll tell him to try that. thanks guys