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Discussion Starter · #1 ·




Greetings Folks - Apologies if this has been covered prior but haven't driven my Stealth much in the last several years so it's been awhile since I've visited this site. Hopefully you guys can help me once again.

1991 N/A Stealth MTX FWD: Owned since 2008. It is my Bonneville 150 Club vehicle: The ECU has been rebuilt before me - at least the caps have been replaced. Unfortunately, it is an OBD I car and my OBD II diagnostic tool does not apply here.

Anyway, the high RPM fuel cut off has always been a little low - as I remember, the nominal spec is supposed to be 7300 RPM. Years ago, the ECU used to interrupt the fuel at around 6900 RPM but now it activates at about 6400. These readings are from the stock dash mounted tachometer. Engine load seemingly has nothing to do with this - it happens at the identical 6400 RPM, regardless if it is at WOT on the road or in my shop in neutral. The engine doesn't miss fire at any other time - it runs great - but once it's at 6400, the engine speed will not increase. The cutoff is abrupt - the critter runs perfectly at 6300. LOL!

So the question is - does the RPM limiter in the ECU degrade over time? If so, what is the procedure to fix it?

I have a fuel pressure gauge taped to the outside of the windshield and it remains pretty constant at about 48 PSI, which is right at service manual spec. The ignition seems to be fine - my spark plug timing light at fuel cut off RPM is still flashing normally. That's one of my, "old school" ignition testing tricks - been using it for decades. LOL!

Thanks - - Any guidance is appreciated -

Jake -
 

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Is it possible that your tachometer has a skewed reading? The simplest way to rule out your suspicions of the ECU is to toss it into another 91-92 N/A (assuming not a base model SOHC Stealth)

A logger/scanner could easily verify the engine rpm too (among other things) short of using the diag. connector underhood.

-sent from my Galaxy Note 9
 

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Discussion Starter · #3 ·
bboyalan - - Thanks for your quick reply. I have thought about an erroneous tach reading - - but have no simple way to check it. Is this a known problem on our early builds? Guess I will take tire diameter/gear ratios/engine speeds and compare them to MPH.

SDSU - in Brookings? My Alma Mater -Ferris State in MI used to play hockey against you guys- a long time ago. .
 

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So the tach and the speedometer on the are both prone to failing capacitors, although the speedo one shouldn't apply as the SLs have a cable driven speedo(or is that just the SOHC cars?) Anyways, yes false tach readings are common.

If I recall correctly the tach has 3 identical capacitors, and all three are needed to have a completely functional tach. If one cap completely fails the tach only moves like 66% of what it should, 2 failed caps is 33%, etc. Now on my tach I had like one failed cap(it was completely popped), and one slightly leaking/failing cap, and my tach was at like 50% of what it should be. So failing caps can have a partial effect, which might be what you have going on.

It's not hard to pull the cluster, it's only like 6 screws and disconnecting some wiring connectors for some buttons, and it slides right out. If you have like a 6th grade understanding of how soldering works you can easily replace the caps yourself as well.
 
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