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Discussion Starter · #1 ·
1992 3000 SL. 104k miles. Has an intermittent miss on light throttle and/or light load (like going uphill). Not consistent. Mechanic drove it for 2 days, never did it. He can't diagnose it because he doesn't have obd equipment that old. Won't change plugs/plug wires because the rear bank is under the intake and getting parts (gaskets, bolts, etc) to reinstall the intake has been a problem lately - his lot is full of other cars waiting for parts. Now I have questions...
1. Does anybody have any ideas regarding an intermittent miss, and 2. Who do you all get to do engine diagnostics?
 

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Panama Green,mostly stock with 13G's, Oohnoo SMIC, money pit
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Wasted 2 days driving it around? Need to find a different mechanic.

Intermittent miss is a pia. But any good tech with a scope and KNOWS how to use it can find that miss.

P
 

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Panama Green,mostly stock with 13G's, Oohnoo SMIC, money pit
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Dude, I read your first post, owning older cars and you don’t some of your own work? Changing plugs and wires isn’t that tough. Not that I promote just changing parts if your not sure but it’s a easy thing to do
 

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Discussion Starter · #4 ·
I agree. His obd scope won't read that far back. I found an imported specialist that can scope it. Got an appointment on the 7th. Meanwhile, I picked mine up this evening, drove it 20 miles home - perfect! I did put sea foam & premium gas in. Interested in seeing if that helps b4 my appointment in the 7th
 

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1991 Dodge Stealth ES 5sp
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The most common issues would be a faulty PTU, Ignition coils (all of them are accessible at the front/side where all the spark plug wires plug in), and lastly an ECU with corrosion.

My issue started off with intermittent misfires. Changed out the ignition coils and PTU but in the end it was the ECU with some corrosion from the capacitors leaking. Cleaned it with rubbing alcohol, which fixed the misfire as long as the car was started cold haha. Ended up replacing the ECU with a reman. unit.

Just pull the ECU, open the case and inspect the board. If it looks crusty/gross, it's probably your culprit.
 

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I give this a 99.9% chance that it's the ECU. For '91-'93 cars, this is the exact symptoms they often exhibit when the caps have leaked and damaged the sub-board that controls injector pulse.

I don't believe it's going to be spark plug related. Those problems usually come out at higher RPM / higher load. As others have mentioned, the PTU and coils sometimes fail. But, in my experience, that's usually a hard failure -- car won't start or has a constant miss on 2 or more cylinders, regardless of RPM or load.

Do yourself a huge favor and start with the ECU. They're ALL bad by now. You CANNOT properly diagnose a driveability issue in these cars until you have a known-good ECU. Even if it's been "gone through" or "recapped", the amount of hacks that take money from people to not repair them right is alarming. I now make it my practice to immediately send out the ECU from any 1G car that I buy. I'd recommend either A) Foreign Auto ECU Repair, out of Texas, Remanufactured Engine Computers w/ 5 yr warranty - 800 241 6689 - They're experts with these ECUs and won't rip you off... or B) James Grieve (he's not on 3Si, but is active in some of the FB groups). He is thorough and knows these ECUs very well, but generally has a long wait list.

Best of luck to you!
 

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Discussion Starter · #14 ·
Just went thru previous owners repair records. The Crank Position Sensor, Ignition Coil & Remanned ECM were replaced exactly 5 years & less thank 10k miles ago.

I did have a whining fuel pump about a month ago. 2 full tanks resolved that but I'm wondering if it pumped some crap outta the tank thats just now working its way thru to the injectors. I did put injector cleaner & 10 gallons of premium in the tank several days ago. Any thoughts?

btw, drove it this morning & no misses
 

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1994 Mitsubishi GTO Twin Turbo MR
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Just went thru previous owners repair records. The Crank Position Sensor, Ignition Coil & Remanned ECM were replaced exactly 5 years & less thank 10k miles ago.

I did have a whining fuel pump about a month ago. 2 full tanks resolved that but I'm wondering if it pumped some crap outta the tank thats just now working its way thru to the injectors. I did put injector cleaner & 10 gallons of premium in the tank several days ago. Any thoughts?

btw, drove it this morning & no misses
If it ain't broke, don't fix it.
 

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1991 Dodge Stealth ES 5sp
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Just went thru previous owners repair records. The Crank Position Sensor, Ignition Coil & Remanned ECM were replaced exactly 5 years & less thank 10k miles ago.

I did have a whining fuel pump about a month ago. 2 full tanks resolved that but I'm wondering if it pumped some crap outta the tank thats just now working its way thru to the injectors. I did put injector cleaner & 10 gallons of premium in the tank several days ago. Any thoughts?

btw, drove it this morning & no misses
It might not solve the issue, but changing out the fuel filter is always good. It's not too difficult and fairly easy to access. Mine was awful.
 

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What I'm reading in your symptoms of a misfire under lighter load, (what we usually call a fishbite type miss) is almost always secondary ignition... When the cylinder firing conditions get to a point of extreme resistance across the plug, (lean mixture, still a lot of timing, and an added load) the spark finds somewhere else to go that has less resistance.
Someone that knows how to use an ignition scope should be able to see the failing ignition components and confirm that...
 

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Discussion Starter · #20 ·
Update! Had new throttle body installed. Resmokesd it & it cleared the code (2437 I believe). Ran great for 15 minutes. Then idle went to 2500 & it started missing like a bitch! Back for more diagnostics...
 
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