'86 stumble/intake backfire under load
Thread Starter
Instructor
Joined: Jul 2005
Posts: 228
Likes: 13
'86 stumble/intake backfire under load
My '86 is suffering from a stumble (un-drivable above idle) issue. The car has about 97,000 on the clock and other than consumables like fuel filters, air filters, plugs, cap, rotor, wires, etc. over the years, its never needed anything.
The car starts up fine and idles fine. It revs ok in neutral. Given throttle under any load, whether driving/moderate normal acceleration or braking against the converter in Drive, the engine with stumble and sometimes backfire through the intake. Return to idle and everything's fine. Happens cold or warmed up. The car will idle down the street in gear fine but as soon as any throttle is needed, it begins stumbling. Which direction should I be looking in? I imagine the fuel pump is probably due soon, but I've only experienced them dying completely, not partially. No codes present.
The car starts up fine and idles fine. It revs ok in neutral. Given throttle under any load, whether driving/moderate normal acceleration or braking against the converter in Drive, the engine with stumble and sometimes backfire through the intake. Return to idle and everything's fine. Happens cold or warmed up. The car will idle down the street in gear fine but as soon as any throttle is needed, it begins stumbling. Which direction should I be looking in? I imagine the fuel pump is probably due soon, but I've only experienced them dying completely, not partially. No codes present.
Drifting
Joined: Jul 2022
Posts: 1,805
Likes: 365
From: Central Florida
Maybe a fuel pump or delivery problem. Idles / starts fine because there is no need for a lot of fuel. Revs OK in neutral, still not a lot of fuel needed. Also check the fuel pressure regulator vac hose to see if the diaphram is leaking gas past it into the vacuum line.
Any codes ?? What is your fuel pressure ??
Have you timed the car recently ??
Make sure timing wire / EST connector is still connected.
.
.
Any codes ?? What is your fuel pressure ??
Have you timed the car recently ??
Make sure timing wire / EST connector is still connected.
.
.
Last edited by Vets-Vet; Jan 30, 2024 at 12:53 PM.
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
I agree with previous post. Bad cap, rotor, wire(s) or fouled plugs(s)
I chased the same as you describe for a year. Ended up being a wire not seated correctly on cap
I chased the same as you describe for a year. Ended up being a wire not seated correctly on cap
Last edited by Cruisinfanatic; Jan 30, 2024 at 08:50 AM.
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Melting Slicks
Joined: Jul 2005
Posts: 2,217
Likes: 302
I'd start with the side that corresponds to the O2 sensor...
When I had mine, it was on the opposite side. I could mildly accelerate, but if I gave it any significant amount of throttle, it would backfire out the intake. But hte ECM wasn't reacting to it because it couldn't "see" what was happening in the exhaust.
However from what the OP is describing, it sounds like the ECM can see it and is probably leaning the mixture out considerably due to unburnt oxygen in the exhaust.... which leads me to speculate that the bum wire/plug is on the O2 side of the engine.
When I had mine, it was on the opposite side. I could mildly accelerate, but if I gave it any significant amount of throttle, it would backfire out the intake. But hte ECM wasn't reacting to it because it couldn't "see" what was happening in the exhaust.
However from what the OP is describing, it sounds like the ECM can see it and is probably leaning the mixture out considerably due to unburnt oxygen in the exhaust.... which leads me to speculate that the bum wire/plug is on the O2 side of the engine.
Thread Starter
Instructor
Joined: Jul 2005
Posts: 228
Likes: 13
Had a warm day today so I got the car out to check it over some more. Plugs look good as does the cap and rotor. The wires, which were properly routed, don't seem to have any bad spots either. I did notice that stabbing the throttle in neutral, from idle to about half pedal or more, would also cause the engine to stumble, buck and backfire through the intake as long as I held it there. Slowly bringing up the revs wouldn't do this. I'm starting to think fuel delivery.