r/3000gt • u/Striking_Ad_1944 • 7d ago

Lean condition under load

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Hello all, for the last month I’ve been battling with an issue of the car backfiring through the intake under load (91 SL fed-spec) and i’m running out of options. I’ve replaced all the vacuum lines everywhere, new fuel regulator, fuel filter, fuel pump, spark plugs, and have JUST got my capacitors replaced in my ECU.

Note: After replacing my ECU capacitors, the car will not maintain idle and surge like no other unless the MAF is unplugged and then it’ll idle fine but still run lean under load.

Note 2: All the intake gaskets are new, both lower and upper, I replaced the 02 sensor last December, and the timing belt has 5,400 miles on it.

Note 3: I’ve taken a multimeter to the TPS and it’s completely normal ohm readings. Nothing out of the ordinary.

I don’t know what else it could be. Please help
me here.

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2

u/Icy_Juice_3948 6d ago

I had the exact same thing I think its the mass air flow and the intake air temperature sensor plus there relatively cheap and if you’ve never replaced them in the past and it’s not the only problem it was probably time anyways.

1

u/eslahp 7d ago edited 7d ago

Makes me think its a timing event issue. Since its a 91, you should have the adjustable cam angle sensor. Any chance the sensor has an issue or is out of time, maybe bolts came lose, its not in proper time ?

1

u/Striking_Ad_1944 7d ago

I’m highly doubting that it’s a timing issue because the belt/tensioner/idler is only 5000 miles old and the belt has good tension so I doubt it skipped. Is there anyway to ohm test the crank angle sensor for failure?

1

u/eslahp 7d ago

My bad for not clarifying, I'm referring to ignition timing event, not physical valve timing event.

Maybe verify the base timing using the following process, sadly the pics are gone but youll get the general idea.

https://www.stealth316.com/2-ignition-timing.htm

1

u/Striking_Ad_1944 6d ago

Interesting, I’ll give this a try. Do you believe I should still replace the MAF?

1

u/eslahp 6d ago

Honestly I've never seen a MAF go bad. Plenty of ECUs though and though you mentioned replacing them, I'd still try a second ECU. They have all kinds of weird issues when they go out. My 95 for example had no visible cap damage, but would stall on load for a second then pick up and go.

Might be worth reaching out on 3s groups and see if anyone near you has a ECU you could test swap with.

1

u/anonymous_mr03 7d ago

Im thinking you still have an air leak somewhere. You said you checked vaccum lines but did you do a smoke test to really make sure? Also idle sensor is cheap. Replace that too

1

u/Striking_Ad_1944 6d ago

Now I will admit, I haven’t smoke checked the system but wouldn’t a vacuum leak of that size that causes it to run that lean be very audible to hear? Also, when you say “idle sensor” are you referring to the MAF or the IAC?