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Discussion Starter · #1 ·
My completely stock '93 turbo starts beautifully and idles smooth, exactly as it should. Immediately after start the engine will rev beautifully with no stumble or hesitation, but in maybe 20 or 30 seconds the RPM's when revved are limited to about 2000, when it just hits a wall and it feels like the fuel is cut until the RPM comes down slightly when it will begin an oscillation from about 1500 - 2000 RPM. It will idle all day or gently rev to about 2000 RPM without missing a beat. The check engine light is not on and there are no codes. What can cause this?
 

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Discussion Starter · #3 ·
My guess would be internal fault with ECU. Might pay to pull cover and look inside for any degrading capacitors or leakage onto board.

jimb
You may have nailed it right off of the bat! I opened the ECU and see a nasty looking little capacitor (leaking brown goo out of the bottom). I'll order a new one to replace it and let you know what happens. Thanks.
Passive circuit component Circuit component Green Hardware programmer Electronic engineering
 

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Good find. Even if the others look ok I’d plan to eventually replace. There is a guide on the forum on sizes required and tips to remove & install. Can be a little tricky as PCB is multilevel and cold solder joints are possible. I stuffed up my ECU and never found the culprit.
 

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My completely stock '93 turbo starts beautifully and idles smooth, exactly as it should. Immediately after start the engine will rev beautifully with no stumble or hesitation, but in maybe 20 or 30 seconds the RPM's when revved are limited to about 2000, when it just hits a wall and it feels like the fuel is cut until the RPM comes down slightly when it will begin an oscillation from about 1500 - 2000 RPM. It will idle all day or gently rev to about 2000 RPM without missing a beat. The check engine light is not on and there are no codes. What can cause this?
Did you mess with your TPS recently? If you install it incorrectly, it will always read the throttle position as "should be idling", and the ECU will start cutting around 1600-2000 RPMs. It's easy to do wrong if you've never read the documentation on proper TPS installation. They expect you to place the TPS on the throttle body 45* back, then rotate it into place and fasten it down.
 

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Discussion Starter · #6 ·
Did you mess with your TPS recently? If you install it incorrectly, it will always read the throttle position as "should be idling", and the ECU will start cutting around 1600-2000 RPMs. It's easy to do wrong if you've never read the documentation on proper TPS installation. They expect you to place the TPS on the throttle body 45* back, then rotate it into place and fasten it down.
That was my original suspicion. I actually ordered another TPS (throttle position sensor) just before I posted my original question. I have not messed with it, however the car has been parked for nearly 8 years and I know how atrophy sets in in many places. I now plan to change that capacitor in the ECU (as soon as it arrives) because it looks obviously bad to me. I will have the new TPS at about the same time, so I will run a test with the repaired ECU to see if it was actually the problem. If that does fix it, I will keep the TPS as a spare. If not, I will at least know that I didn't mess anything up in the ECU, and will the go forward with the TPS. Thanks for the advice on its installation. I did see it recommended in the BGM (Big Green Manual for any new comers).

By the way, I am seeing a pattern that looks like capacitors going bad is one of the most major maintenance issues. Both my tach and speedometer are not working after this long span of inactivity. I have the gauge cluster out now and will change the capacitors in them tonight.
 

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Discussion Starter · #7 ·
Still having the same problem (RPM limited to about 2000 AFTER warm up). Before warm up (from a cold start) it starts easily and idles smoothly. Initially it revs like it should, but when warm it hits that low RPM threshold and drops to idle. Then when holding the throttle up will rev back to 2000 (or so) and repeat but never exceed that speed.

I have now changed the TPS and reset per BGB specifications. Changed the temperature sensor in the inlet housing and replaced 3 suspicious capacitors in the ECU. No change in symptoms. I don't know what else to try myself, so I guess my next move is to send my ECU to zkboys (found on ebay) for diagnosis and repair. Does anyone have any new thoughts for me. I have this beautiful car that is begging to be driven, but not with this issue.
 

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I'd be inclined to replace the remaining caps as it can very difficult to tell if they are faulty. Other than that there could be damage to the board or IC's due to cap faults. If you could either buy a replacement ECU or know someone kind enough to loan you one to test that would be ideal. I wouldn't really trust a company like zkboys to repair & test unless others have had good experiences. Seems like dead money as the ECU's are well past use by date.

Another option is to take the plunge and buy an EMS. There are some good choices for PNP models that make installation & tuning a breeze.
 

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Discussion Starter · #9 ·
Once again, sounds like a good suggestion to replace the remaining capacitors. I am comfortable with soldering and circuit boards and if this works will be much less expensive than buying a new ECU. I would just as soon keep mine as original as practical since it is a completely original and non-molested car. And the ebay deal is completely out of the blue and I don't have any idea of their actual capability for testing or their success ratio.

If anyone out there has a known, properly functioning stock ECU (89661-17390) that will either sell reasonably or rent to me for a test, I would be interested.
 

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Discussion Starter · #10 ·
YAHOO! I ordered up the full set of capacitors for the ECU from Digi-Key Electronics and finally spent the 3 hours that it took to replace them all. It's back! My old turbo girl feels like a kid again. No more problems. This reinforces the concept that capacitors go bad with time. Initially I replaced two that seemed to show signs of leakage, but they weren't the culprits. These capacitors are not difficult to replace with a little patience and basic soldering skills. It does take knowing how to de-solder connections from the board (my favorite is to use copper de-soldering braid to wick up the melted solder from the back side). Thanks for the good advice.
 
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