Hi my '90 z28 has been acting up since last year, and I'm getting fed up with trying to get it running right. Here's the summary:

When idling, in neutral and sometimes in park, the car revs erratically. It will rev high and then drop low, to the point of stalling sometimes. The lights also dim and brighten accordingly, and the car will move forward if I don't have the brake clamped. While driving it does the same thing only very slightly, but I notice it

I have changed the following, new air intake, cleaned the throttle body (/w new gaskets) all fluids changed, and used a (i forget what its called it cleans the fuel system, you add it when you gas up) thing. But to no avail.

Help is desperately needed to get my baby running right.

Thanks in advance.

 

When you say Clamp the brakes thats got me thinking brake booster leak or vacuum leak but Check all vac lines,tps ,iac, is the check engine light on?

 

Definetly start by checking all your vacuum lines. Look for and replace any cracked/splitting lines. Also, spray carb cleaner or starting fluid over all the vac lines, the egr, the plenum (assuming its tpi), all gasket areas. The egr gasket and intake gaskets are famous for having vac leaks. If you spray either of these on a vac leak you'll notice the idle change because its sucking it in. Check for leaks first.

If not this, have your alternator checked. I say this because you say the lights go dimmer and brighter as well, this could be a sign of your alt going bad or a bad connection somewhere. Either way, i would say check for vac leaks, if you find any replace them. If that fixes it, good! If not, well they needed to be fixed anyways :P If that doesnt fix it, then take your alternator to somewhere like autozone to have it checked.

Let us know what you find

 

Ya thanks alot I know I did have some vac leaks and I thought I checked them and replaced some, but I'll check again and let you know.

And I actually had my alternator replaced about 2 years ago, so I doubt that's it but I'm willing to check anything.

Ill keep you posted.

 

When the idle drops so low the car is almost stalling, the alternator is barely turning, it will make the lights dim a bit. Pretty normal.

Surging idle conditions are a result of the ECM being confused by its sensors. One of them is acting up. Is it throwing any codes?

Does it do it more or less when it's hot or when its cold?

DEFINITELY check for vacuum leaks as they WILL cause this. A diagnostic vacuum gauge from an auto parts store is $20 or so, would be good to see what kind of vacuum it's pulling. The idea to spray carb cleaner around the vacuum lines isnt a bad one, but be careful as that stuff is flammable (keep it away from exhaust parts). You can do it with water too, actually.

Anyway your car is speed density, so no maf to worry about. So most everything it could be is a cheap fix, you just have to find which part is acting up.

The ECM gets information from all of its sensors, and if one is showing a reading of X that is too low, the ECM will adjust the settings to bring that value into the correct range. As it does this, another sensor will read some other parameter, and give the ECM an anomalous reading of Z on Parameter 2, so the ECM will adjust that back into range. This results in a seesaw effect. To bring one sensor into the correct range, the other one will go out of range, because one of them is wrong, and the ECM has no way of telling which one. It's compensating for what it's being told, and in the process the idle goes up and down as it adjusts.

Vacuum is a parameter read by the MAP sensor and if it's low, it throws off everything the ECM does. So in that case, even though all the sensors are functioning properly, your vacuum is lower than it should be and the ECM sees that and tries to fix it the best way it knows how. In that case it's a mechanical problem. So check vacuum first.

So find the cause. Could be a CTS, Map sensor, o2 sensor, vacuum lines, etc.

 

Alright so, this morning ive been looking into the vac hose issue, and I feel like I'm missing something, to be honest I'm not even sure where all the vac lines are on my car, if anyone knows which lines might be the culprit and their location that would be helpful.

A few more symptoms, it really only starts rev-ing strange after its been warmed up for a few minutes, and it seems the longer I run it the worse it gets. But it seems kind of random as well.

Any more help would be appreciated.

 

O2 sensor?