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420A 420a idle issues after ATX to MTX swap

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steve1559

Probationary Member
9
0
Mar 21, 2022
Salisbury, Maryland
did a successful automatic to manual swap on an Eclipse 420a 1999 but am having serious idle speed issues (car revs way too high at traffic stops---2500 to 3000 rpms---unbearable! Otherwise runs and shifts great----lot more fun. Soooo does anybody know what could be causing this? Things I've tried: new idle control switch, cleaned throttle body, checked for vacuum leaks. Someone suggested the head gasket could be compromised and causing problems. Anybody who's been down this road before your input would be greatly appreciated. Thanks
 
What's the idle like when you start it up?
Thanks for the response. At the moment of start up it immediately revs to 3000 and almost immediately drops back down to 1000 which is normal. Then after driving around for a few minutes and after several traffic stops it begins to surge (up to 3000) and eventually stays there if you continue driving.
 
Headgasket would not cause this issue.
Thanks for the response. I offered this possibility because a mechanic briefly looked at it and thought it had a blown headgasket which he said would act like a vacuum leak. He also noticed that it seemed to be running hot mostly because air could be seen and heard bubbling up in the plastic coolant reservoir. We told him the dash temp gauge needle does not climb to indicate overheating. He still thinks head gasket. Thanks
 
Have you checked your throttle cable adjustment? I had a similar issue (minus the bubbling coolant in the overflow) after my rebuild. Idled fine after startup, but once I got it out on the road, the idle slowly made its way up over 3k. Pulled over, popped the hood, loosened the two throttle cable bolts on the intake plenum, pushed the throttle plate closed (which immediately lowered the idle), tightened the bolts back down and was on my way.
 
Wow! Thanks for the response and the suggestion. Will definitely be applying that tip and let you know how we fair. Certainly will be a simple fix if it works. Some folks are insisting the factory electronics, sensors, and computers on cars since the 90's are too factory programmed to accept something as radical as a
Have you checked your throttle cable adjustment? I had a similar issue (minus the bubbling coolant in the overflow) after my rebuild. Idled fine after startup, but once I got it out on the road, the idle slowly made its way up over 3k. Pulled over, popped the hood, loosened the two throttle cable bolts on the intake plenum, pushed the throttle plate closed (which immediately lowered the idle), tightened the bolts back down and was on my way.

tranny swap modification---confuses the ECU/TCU, etc. Hope it makes a difference and will let you know either way. Big thanks.
 
... a mechanic briefly looked at it and thought it had a blown headgasket which he said would act like a vacuum leak. He also noticed that it seemed to be running hot mostly because air could be seen and heard bubbling up in the plastic coolant reservoir ...
If air is bubbling up in the coolant reservoir I think you've got a head gasket problem. It's not really 'air' of course -- it's exhaust gas. And mixing any gas with your coolant means it won't cool as well so overheating is very likely.
 
I'd conduct a compression test and check the condition of the spark plugs before coming to a conclusion of a headgasket issue. But before even doing that I'd figure out the revving too high issue. They are likely separate issues and I'd get it running right first.

Another thing to note. The stock temp gauge is what we call a "dummy gauge". Just a very broad idea of what your water temp actually is. So just because it's not reading at the top doesn't mean you not running slightly hot. Typically in the early stages of cooling system issues it won't overheat much unless you're running it hard. And by the way the reservoir is where the extra pressure gets sent when the 420a runs hot. I know my headgasket is sealed nicely but when I ran low on coolant I could hear pressure rushing out of the reservoir cap.

So, said simply, I'd do some diagnosing and try to figure out what exactly is going on with the car. Randomly jumping to conclusions usually ends up with spending unnecessary money. And the good ole "my mechanic said"... Well honestly they'll say whatever puts a check in the business. Not all are like this, but it is better to be sure first.
 
I'd conduct a compression test and check the condition of the spark plugs before coming to a conclusion of a headgasket issue. But before even doing that I'd figure out the revving too high issue. They are likely separate issues and I'd get it running right first.

Another thing to note. The stock temp gauge is what we call a "dummy gauge". Just a very broad idea of what your water temp actually is. So just because it's not reading at the top doesn't mean you not running slightly hot. Typically in the early stages of cooling system issues it won't overheat much unless you're running it hard. And by the way the reservoir is where the extra pressure gets sent when the 420a runs hot. I know my headgasket is sealed nicely but when I ran low on coolant I could hear pressure rushing out of the reservoir cap.

So, said simply, I'd do some diagnosing and try to figure out what exactly is going on with the car. Randomly jumping to conclusions usually ends up with spending unnecessary money. And the good ole "my mechanic said"... Well honestly they'll say whatever puts a check in the business. Not all are like this, but it is better to be sure first.
Thanks for the response! We're definitely going to confirm the headgasket possibility BEFORE pulling the cylinder head off. Going to pull the plugs too to see if they're too 'steamed clean', etc. Hope to keep you updated as we trouble shoot.
 
thanks, dsm community, for your quick input and response to our little crisis with the unbearably high idle after the AT-to-MT swap. the culprit ended up being a massive vacuum/air leak in the EGR tube ..(especially the end that mounts onto the lower part of the cylinder head) We cannibalized another tube with healthy gaskets off a parts car and it made the air tight seal. Instant fix! We identified this by spraying copious amounts of starter fluid all over the engine as it was idling to find out where it was leaking. Well, it went wild when we sprayed around the area where the EGR tube bolts onto lower area of the head. Again, big thanks.
 
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