Rolling Idle

[FEARNONE30]

I have a 91 sho mtx and it seems to have a rolling idle after I push the clutch in after cruising in gear. What I mean is it rolls from 1000 rpms to 1500 rpms and then settles. This car had the valve adjustment, valve cover gaskets and well seals, plugs(agsp32pp), and wires. I also did the timing belt, alternator, belts and hoses and did the idle reset as per Spdatt. Any help would be great.

 

[Mike Kopstain]

A good start is to check the codes. Do that by clicking here.

First instinct is the Idle air bypass valve. Over time these get clogged with carbon and are difficult to move or the motors get weak. Either way, on an older car you will find an erratic idle.

The second thought is the VSS. I'd guess that 99% of VSS problems go undiagnosed on the MTX SHOs, since a faulty VSS won't affect shifting. It does however affect idle speed while rolling. That might be another place to look.

 

[rendyx]

I have had such a problem. Check your IAB, as Mike said, as it has caused me many troubles. Make sure that the gasket around the IAB is in good condition too. I made my own from RTV, as a new one is hard to find. Another thought, my SHO gets confused on it's idle when I turn on the defrost or A/C, as the compressor is disconnected.

 

[FEARNONE30]

Thanks guys, I guess it's time to tear that ugly monster called an intake apart and give it and the accessories the cleaning of a lifetime. Once again, thanks. If it weren't for this forum, I would not own these cars.