90 SC power loss WOT

Tunevo

Registered User
Wondering if anyone had had this issue before. 1990 SC, runs great when cold (open loop), once it gets to.operating temp and you drive about 2 minutes, at WOT it falls flat and backfires at 3000 RPM. It will run fine at WOT when at temp for the first 2 minutes after a startup. So pull over key off, wait 2 minutes, it's ok again then 2 minutes later, same issue. I can get to 4500-5000 RPM driving normally, just no WOT. No CEL on dash, however, when running KOEO / KOER, I have codes coming up. 25 - Knock Not Sensed During Dynamic Response Test (R), 66 - , MAF signal below minimum test voltage of .4vdc (O,M,R), Transmission Fluid Temperature Circuit Grounded (M,O, 72 - Insufficient Manifold Absolute Pressure/Mass Air Flow Change During Dynamic Response Test (R)
73 - Insufficient Throttle Position Change During Dynamic Response Test (O,R)
74 - Brake On/Off Circuit Open–Not During Self Test (M,R) and 87 - Primary Fuel Pump Circuit Failure (M,O)

I want to say MAF, but issues didn't occur until I changed out the fuel filter. However, MAF could be bad if now it's overwhelming with fuel woth the new filter at WOT. Would this make sense as a diagnosis to my issue? Or does anyone else have any other probable cause? All sensors and coils, DIS, plugs etc are all new. Just MAF is old.
 
Update, MAF test key on, 0.01V, test at idle 0.75V, does not seem to be reading poorly. Would TPS cause this type of condition as well?
 
Would TPS cause this type of condition as well?

What TPS supplies is a session-based reference signal based on sweep/min/max. Fails would show 23, 53, 63 codes, which you haven't reported, so...

The MAF 66 is curious. Some of the codes indicate the tests may not have been run properly. As an example, 72...which if not present when re-reading tests might get rid of the 66.

I'd want to know if the MAF signal volts rise/fall in a repeatable sweep from idle to rev up.

As for your WOT stumble, I'd generally suspect plug wires/plugs, then fuel pressure (if you pull the vacuum line off the FPR, can you detect any raw gas?), then, as a reach, plugged cats. If you have a laser thermometer, check them to see if they are both similar temps, how high they read after a drive, and listen for rattles that indicate failing honeycomb inside.
 
What TPS supplies is a session-based reference signal based on sweep/min/max. Fails would show 23, 53, 63 codes, which you haven't reported, so...

The MAF 66 is curious. Some of the codes indicate the tests may not have been run properly. As an example, 72...which if not present when re-reading tests might get rid of the 66.

I'd want to know if the MAF signal volts rise/fall in a repeatable sweep from idle to rev up.

As for your WOT stumble, I'd generally suspect plug wires/plugs, then fuel pressure (if you pull the vacuum line off the FPR, can you detect any raw gas?), then, as a reach, plugged cats. If you have a laser thermometer, check them to see if they are both similar temps, how high they read after a drive, and listen for rattles that indicate failing honeycomb inside.

Thanks for the reply. I know the codes don't seem right as why I'm reaching out. MAF rises and falls repeatedly on and off throttle. Ran 2 wires inside car connected to my multimeter, I don't see an issue with it so far. Exhaust was replaced with new o2's, brand new 2.5" high flows, new resonator, MBRP mufflers. New plugs and wires, new coil, new DIS module.

I did meter my tps and at WOT is reading 2.01 and slow climbing, 0.04V at rest position. Maybe I'll try replacing the TPS again and reset the EEC and see what happens from there. Vacuum is good at 20 at idle, no leaks, regulator was also replaced in March when fuel pump was replaced. I'm pulling at straws, but will see if I can also check that I didn't get a bad regulator.
 
>I did meter my tps and at WOT is reading 2.01 and slow climbing, 0.04V at rest position.

If those are reliable/repeated TPS numbers, I'd expect codes. What do you mean replace it again? Sounds like something else is going on and just throwing parts at it won't help.

Example of acceptable values: .956v throttle closed; WOT 4.13v (these are just examples that don't warrant action). Point is those represent a good reference range. Your numbers certainly don't, but if they represent what the EEC has to work with, I'd expect codes.

I'd repeat code read before I replaced the TPS, being careful to warm the engine, then a full healthy gas pedal to the floor when told to goose test. It may be helpful to remove the TPS and check it's voltages while you rotate it by hand.

Are you sure you're getting full throttle out of the pedal/linkage? Noting that if all you had was part throttle, it still shouldn't stumble. Have someone floor the throttle, engine off, while you witness the linkage and/or look inside at the throttle plate to confirm swing.

What plugs did you use? What gap?
 
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That's my thought as well. No CEL's at all and those are 100% WOT values on the TPS. Put my spare in and values are 0.8V and WOT 4.73V. So that's one. Now have a bit more to work with as nownive gained 500 RPM before the same power loss encounter, just not as hard. Plugs are autolite platinum gapped to .054 (2 were closer to .052). It is verified reaching full.pedal travel. I can drive around normally still to a max of 3/4 pedal. Anything more warrants the car falling on its face until I back to 3/4 pedal and resume. Just did a good 800km trip with it last week to Toronto and back, no issues other than WOT issue or over 3/4 throttle. Could it potentially be a failing EEC, or again maybe just a weak plug?
 
Also to add, car is fine for the first 2-5 minutes of driving, even at WOT it pulls hard, then like I was saying if won't pull anymore. Soon as something heats up it acts up.
 
I'd put the spare TPS back in, then relearn the EEC.

Plugs are autolite platinum
double plats, you mean?

I can drive around normally still to a max of 3/4 pedal. Anything more warrants the car falling on its face until I back to 3/4 pedal and resume.
Sometimes an issue like that is when the intake (tube) is collapsing under higher vacuum, or plugged air filter, etc.

Is the bypass on the supercharger working ok?

Sorry if I missed it - what were the original issues that led to this work? Any mods?

Are you still getting WOT when cold? It shouldn't take long to leave open loop, I think, so I'm a bit confused how you're able to reliably test that.
 
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I'd put the spare TPS back in, then relearn the EEC.


double plats, you mean?


Sometimes an issue like that is when the intake (tube) is collapsing under higher vacuum, or plugged air filter, etc.

Is the bypass on the supercharger working ok?

Sorry if I missed it - what were the original issues that led to this work? Any mods?

Are you still getting WOT when cold? It shouldn't take long to leave open loop, I think, so I'm a bit confused how you're able to reliably test that.

Well spare TPS is in, relearn is done, drove around city/highway for 20 mins or so, when engine at operating temp I can reach redline no problem WOT, wait 5 minutes I can reach 4000RPM, after 10 minutes 3000RPM max at WOT. Shut the car down, wait 1 minute, can hit WOT for about 30 seconds and it's back to the 3000RPM power loss. Came back ran KOEO, no codes, ran KOER, no codes. I'm lost now as I have no error data. Air filter is new, intake tube is fairly rigid, I sense this area is ok. Supercharger bypass is opening fully, also registering 10psi boost up until it hits 3000RPM. only work I have done before this happened was changed the fuel filter. I'm now thinking fuel pump once it gets warm is crapping out, or sock is excessively dirty (which it shouldn't be). Also may as well pull the plugs (yes x2 platinum autolites, sorry) and see if there is a lean burn or signs of a bad firing plug. Might be my last few options.
 
So after dinner update, still no KOEO codes, but, have KOER codes 72 and 73 again. MAF and TPS. I'm thinking there is something wrong there.
 
only work I have done before this happened was changed the fuel filter.

I'd revisit it. Take it out, confirm correct P/N, not plugged, proper orientation, take it back for another one if needed.

If you can get your hands on a fuel pressure gauge and attach it at the rail, you can see if things are OK pressure wise. See if you can safely locate it so you can watch it while driving.

We had a snowcat that had the same symptoms one time - it would run ok until it hit a steep slope, then the rag would block the pickup. Seems the operator wanted to go home early...

Any chance an angry neighbor stuffed something into the fuel tank? Did you use a floor jack to lift the car while changing the filter, maybe lifted on the tank by mistake?
 
I'd revisit it. Take it out, confirm correct P/N, not plugged, proper orientation, take it back for another one if needed.

If you can get your hands on a fuel pressure gauge and attach it at the rail, you can see if things are OK pressure wise. See if you can safely locate it so you can watch it while driving.

We had a snowcat that had the same symptoms one time - it would run ok until it hit a steep slope, then the rag would block the pickup. Seems the operator wanted to go home early...

Any chance an angry neighbor stuffed something into the fuel tank? Did you use a floor jack to lift the car while changing the filter, maybe lifted on the tank by mistake?

Spare TPS tests good. MAF at idle 0.37V WOT 3.91V tested while driving on multimeter. Maybe I'll try putting old gas filter back in and try that. Can't hurt at this point. And fuel pump was done prior to this issue I'm having so not sure if the pump is on it's way out or was given an N/A pump and it just can't support the car under load anymore from too much demand. These seem to be my go to points for now. Will report back.
 
Yeah, I'm stumped, going to try a fresh set of plugs and close the gap to .050 and see if that helps, if it's not a cool, then I'll try plugs again... maybe I got one that's fouling out under boost, or a wire that's resisting too much. If that's not it, I'm dropping the tank again. Worth a shot I guess, stranger things have happened hahaha. Thanks for the input, I'll post it I get it figured out
 
I ended up sticking to a .052 gap and no change. However it finally threw a code at idle... code 66 MAF voltage below 0.4. So I'm going to order a new MAF. When car idles warm I register 0.54 after it bogs I pull over and measure 0.33. WOT on bog condition is 3.55.

Also I know it's not a spark or ignition issue, plugs are golden brown so its stoich running perfect fuel, and when it goes this big condition, I leave the car in 2nd gear, shut it off, turn ignition back on and it'll pull hard again. So it's an electronic issue. I think now the MAF is my trigger after all.
 
So... still having issues. What are the signs of a failing EEC? MAF voltage was showing low when condition encountered, swapped MAF and same thing good reading when warm, then drops reading from 4.8V WOT to 3.51V WOT. 2 MAFS same problem, only 1 is brand new. Same with TPS, readings good when cold, warms up and signal at WOT not coming up to voltage. Just replaced alternator as it died last week and battery also has a dead cell. I'm looking for ideas as to what is causing this voltage loss to the sensors after car is running. Like I said, when warmed up but driven for a few minutes things are fine, after that no power and refuses to go past 2800-3000 RPM at 3/4 to WOT. I'm losing my mind hoping it's not the EEC.
 
Just replaced alternator as it died last week and battery also has a dead cell.


Bit late, perhaps, but just a headzup - resist using the SC's alternator to recharge a dead battery...that's a quick way to quick alternator death. If the battery is dead, put it on a charger first.
 
Bit late, perhaps, but just a headzup - resist using the SC's alternator to recharge a dead battery...that's a quick way to quick alternator death. If the battery is dead, put it on a charger first.

Yes, as always. I trickle charged it last night, changed the alternator this morning. Would a bad cell battery cause electrical gremlins? I'm at a loss as to what's messing up my electronics signal. I had put 2 new 4GA grounds on the car, 1 to chassis, 1 to engine, so that should eliminate grounding issues. I can't figure this out for the life of me and no CEL issues or codes at all.
 
Would a bad cell battery cause electrical gremlins? .

Not sure we've gone over it in this tread yet, but anything that results in less than, say at least 12.7 volts, will raise heck with an SC's nervous system.

I'm at a loss as to what's messing up my electronics signal.

EMI, for starters, typically from compromised/ad-hoc wiring harnesses.

I had put 2 new 4GA grounds on the car, 1 to chassis, 1 to engine, so that should eliminate grounding issues.

Depends.... In the example where the accy. bracket & engine/heads have been painted, you can add grounds all day long, but if the DIS doesn't see a proper spec ground back to the battery, all bets are off. Another is when the EEC has been out and the center bolt hasn't been re-installed...it's the ground there.

Adding grounds is a good troubleshooting tactic, when used correctly, but there is nothing that will replace the factory ground paths. Confirm them whenever possible/replace if in any doubt...corrosion loves to hide out of sight. EVTM is your friend in that case, and the factory Engine/Emissions Diagnosis shop Manual is next.
 
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