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12/11/15 7:52pm - Original Message: '1994 vette runs but idles rough when hot'
Dans vette
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Hartselle, AL - USA

Vette(s):
1994 C4 corvette

Joined: 12/11/2015
Posts: 11
Hello, I just purchased a 1994 corvette with 69,000 miles on it. It runs great, but after it gets hot, or sits a little bit after running, the idle goes from normal to about 1500 RPM's and goes back to idle. It kind of surges and goes back normal just sitting there with the brake on, in drive. I'm not getting any codes or service engine light, my mechanic has changed numerous parts with no improved results. We thought it had to be the ECM unit, sent it off, got it back in and still are having the same problem. He hasn't changed the distributor but thought I would check around to see if anyone's had this problem before I start just changing parts and spending money. Any thoughts would greatly be appreciated. Thanks!

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12/12/15 8:27pm - Reply: 'Re: 1994 vette runs but idles rough when hot'
PoloGreen93'
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Vette(s):
1993 coupe 100% original polo green

Joined: 7/26/2015
Posts: 34
Hey there Dan this is also a guy named Dan my 93' was acting the same way about a yr ago , a machanic friend suggested I clean the I A C ( idle air control valve ) . Mine is 5.7 300 hp it located at. The beginning of Intake manifold , has a 2 wire plug on it and a black plastic cover takes a 15/16" wrench or cresant wrench , it was dam tight so a little PB BLASTER and patience and it came out cleaned it with carb cleaner a couple of times . Fixed my problem , hope this helps
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12/12/15 10:37pm - Reply: 'Re: 1994 vette runs but idles rough when hot'
Dans vette
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Hartselle, AL - USA

Vette(s):
1994 C4 corvette

Joined: 12/11/2015
Posts: 11
Thanks for the reply but I think my friend already cleaned mine and even had a spare that he swapped it out and tried but with no luck. We replaced the EGR valve that needed replacing. He had some spare parts laying around that he knew were good and swapped different parts out to try to fix it, but none worked. That's when I sent the computer off to be checked. Thanks for the reply though, we might just clean it again.

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12/13/15 3:14pm - Reply: 'Re: 1994 vette runs but idles rough when hot'
Tesla220
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Joined: 4/23/2015
Posts: 64
Service the Throttle Body?   Also scan the coolant the coolant temp (for the ecm, not the guage) and see if it goes low when the idle goes high. Mine was doing that and a thermostat fixed it.

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12/16/15 12:34pm - Reply: 'Re: 1994 vette runs but idles rough when hot'
Warlock
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Vette(s):
1989 black convertible C4 corvette 4 speed automatic 350 CI with 300 Hp. New mods will bring it to 375 Hp.

Joined: 2/1/2015
Posts: 10
It's the opty spark system or fowled injector
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12/16/15 11:19pm - Reply: 'Re: 1994 vette runs but idles rough when hot'
bigfoot68
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Aurora, CO - USA

Vette(s):
1994 Admiral Blue

Joined: 5/15/2014
Posts: 2
Hi. I have a 94 with 80k miles. I had the same problem. You said you sent the ECM out. Was it replaced or just checked?
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12/18/15 11:27am - Reply: 'Re: 1994 vette runs but idles rough when hot'
Dans vette
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Hartselle, AL - USA

Vette(s):
1994 C4 corvette

Joined: 12/11/2015
Posts: 11
I couldn't get a new or re-manufactured ECM anywhere, even through the Chevrolet place. My order was cancelled from their parts house. I had to take my ECM out and let the Chevy dealer send mine off and be rebuilt for $188, which took about 10 - 14 days, but that still didn't solve my problem, but now I know the computer is ok. I think it's the opti-spark (distributor) that's going out. It's located under the water pump. The water pump has to be pulled to get to it. Mine is being replace right now and putting on a new water pump since they have to take it off anyways. I'm waiting on a window guide for my passenger window thats broke and needs replacing too. When I get the car back I will let you know if the opti-spark fixed it.

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12/18/15 2:58pm - Reply: 'Re: 1994 vette runs but idles rough when hot'
bigfoot68
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Aurora, CO - USA

Vette(s):
1994 Admiral Blue

Joined: 5/15/2014
Posts: 2
I hope the Opti replacement fixes it. I replaced mine with a MSD unit. And the water pump too. Unfortunately for me that was not the cause. Mine was heat related with the ECM. Let me know how it works out.
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1/3/16 8:10pm - Reply: 'Re: 1994 vette runs but idles rough when hot'
Dans vette
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Hartselle, AL - USA

Vette(s):
1994 C4 corvette

Joined: 12/11/2015
Posts: 11
The distributor and opti-spark was replaced and I'm happy to say that after diving about 500 miles so far, it's doing great. I'm about positive that's what the problem was.

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10/6/16 8:17am - Reply: 'Re: 1994 vette runs but idles rough when hot'
Warlock
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Vette(s):
1989 black convertible C4 corvette 4 speed automatic 350 CI with 300 Hp. New mods will bring it to 375 Hp.

Joined: 2/1/2015
Posts: 10
Hello, I had the same issues and pulled codes by shorting out pins A & B to get codes 22 and 42 which are the TPS sensor replacement, Electronic spark control module and ignition coil pack. It now runs great and power / fuel economy increased after installation.
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