90 Corvette L98 manual trans.


This has been an intermittent problem over the past several months. It's just a recreational vehicle so I don't really pound the miles on it. Matter of fact, I'll just drive it locally because of this.

Cold start. Starts fine, idles fine, everything works except my radio antenna. I have it in the up position.


After about a 20 minute warm up , the idle deteriorates to rough, The A/C controls get locked in position, the radio sticks to whatever station and volume you had before on the glitch hits. All the controls of the radio and temp conrol no longer operate. No on/off, no up down temp. Just frozen. Meanwhile the idle is bucking. It works fine under acceleration and has plenty of power. Stop at a traffic light and the engine sounds like it's surging up and down, up then down. The belt tensioner at this point is dancing up and down too.
There are no warnings or lights on but there is a code in the display next to the temp up/down control button where the outside temp usually displays. It show a ( - -- -- ). Short dash, long dash, long dash.


So I'm just

Any thoughts on this are truly welcome.

Sounds like an o2 sensor or temp sensor, but you should get a code.

Maybe the IAC valve? I don't know why that would cause the electronics to go haywire though...

Prom issue perhaps, becoming unseated. ECM going into limp mode. CCM losing connectivity to the ECM, maybe freaking out? Though that is not supposed to happen.

Also could be a power regulation issue, maybe the alt is junk. There is some voltage correction stuff in the bin that effects injector pulsewidth, that could account for the strange running condition and if the voltage drops your CCM could be malfunctioning.

-- Joe

So....

Poor performance once warmed up is usually issues with closed loop (vacuum leaks, bad O2, ignition control or ECM), but the car accelerates fine..

Electronics issues usually voltage or grounding

Jumping tensioner is worrying


Put all three together - alternator just about to seize?

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It definitely sounds like a low voltage issue, could also be the tensioner spring is broken so alt is not able to charge it enough due to slipping belt.

I am thinking something is amiss with the engine controller and CCM. The CCM helps the climate control... along with the audio system.

I wonder if a mousie has gotten in there and dined on some wiring.

The strange thing is I went out 3 hours later and the controls were back again. Mouse not probable but there are a few roof rats running around. That is possible. They like to get up into cars too. I have 6 rat bait traps lying in different places around the yard. Oh woe is me. Woe is me. ...........Riggin friggin molten ratten sumbitch.

So far, I have to replace the antenna, change the tensioner, check for vermin and try to figure out some of the above and there probability. Keep em coming. I'm open for suggestions.

Tommy, first thing I would check is the battery/charging system. After that, like Andy said, I'd be checking the CCM and wiring.

Matthew

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That was my first thought. I would measure with a voltmeter when the car is warm. Don't trust the dash guages.

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Jumping tensioner could simply be a result of a weak spring and the engine running rough.

I know right before my alternator went caput the gauges went crazy, the digital dash was flashing on and off. The car stalled. All was well after the swap. Just a tought.

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My tensioner jumps up and down quite a bit when the AC compressor is engaged. Doesn't move much at all till the AC compressor kicks in. All works fine though so I never messed with changing anything. It has been this way since I got the car 2 years ago. I wouldn't worry about the jumping tensioner till you get the other things worked out as I'm pretty sure it isn't a problem.