Won't go over 4000rpm - Problem Solved
#1
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I have an '81 Auto that I've brought back to life. It was a non-runner when I got it. I had to make a new front engine wiring harness, replace all the vacuum hoses, replace lots of sensors, ignition parts, fuel pump, relays, and a lot of other projects. I passed CA smog testing. It idles fine and pulls well from a start but the power starts to disappear about 3500rpm and just goes flat at about 4000rpm and the rpm won't go any higher. After researching here I figured it was a plugged cat. I did the vacuum test suggested at Nichols' site and it passed with a steady 20 inHg at 3000rpm. Anyone have any ideas about what else I should check?
Last edited by Boatman; 09-08-2011 at 03:53 PM. Reason: I found the problem
#2
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Some random possibilities...
- Ignition timing way off. Sounds unlikely, because it should be down on power even at low RPM.
- Running out of fuel. Plugged tank filter, faulty in-tank pump, low fuel pressure, bent fuel line, etc.
- Running out of air. Intake air flap not opening all the way, rag in the intake, faulty air filter.
- Choked exhaust. As you assumed, the manifold vacuum should drop off as the RPM go up. You really need to have the engine under load for a definitive test. You can run a hose into a side window to a vacuum gauge. Unlikely to be the problem, as I would expect the vacuum to have dropped by 3000 RPM.
- Cam timing way off.
Any symptoms, such as running rough when it loses power?
- Ignition timing way off. Sounds unlikely, because it should be down on power even at low RPM.
- Running out of fuel. Plugged tank filter, faulty in-tank pump, low fuel pressure, bent fuel line, etc.
- Running out of air. Intake air flap not opening all the way, rag in the intake, faulty air filter.
- Choked exhaust. As you assumed, the manifold vacuum should drop off as the RPM go up. You really need to have the engine under load for a definitive test. You can run a hose into a side window to a vacuum gauge. Unlikely to be the problem, as I would expect the vacuum to have dropped by 3000 RPM.
- Cam timing way off.
Any symptoms, such as running rough when it loses power?
#3
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Timing is good - that is part of the smog test. Plenty of fuel in the tank. I don't believe my model has an in-tank pump. Fuel lines are good, but I haven't checked fuel pressure. I had the intake system all apart and everything looked good. New air filter. It doesn't run rough it just seems to run out of power. If I'm up around 400rpm and I stab the throttle in downshifts but the engine just won't spin any faster. The linkage is all good and the wot switch is working.
I haven't checked the cam timing though. Could it be that far off and still run fine at lower rpm? Well enough to pass the CA smog police?
I haven't checked the cam timing though. Could it be that far off and still run fine at lower rpm? Well enough to pass the CA smog police?
#4
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The distributer set a little clockwise past the center of the adjustment range.
The firing order is good (the distributer cap is numbered.)
I'll have to get a gauge and attachments to check fuel pressure, but how would I check for sufficient flow?
The firing order is good (the distributer cap is numbered.)
I'll have to get a gauge and attachments to check fuel pressure, but how would I check for sufficient flow?
#5
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Center of range makes sense, so not a tooth off on distrib install.
Basically with a catch can, but with safety precautions.
Pull the cam gear covers and #1 plug to check harmonic balancer for rotation and to check cam gear back notches match covers which in turn match the harmonic balancer.
BTW, makes sense to confirm the cap matches firing order, at one point there were mislabled caps and a batch of wrongly printed firing order stickers, though I don't think it was for the 81.
Basically with a catch can, but with safety precautions.
Pull the cam gear covers and #1 plug to check harmonic balancer for rotation and to check cam gear back notches match covers which in turn match the harmonic balancer.
BTW, makes sense to confirm the cap matches firing order, at one point there were mislabled caps and a batch of wrongly printed firing order stickers, though I don't think it was for the 81.
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I’ve had trouble in my 80 US 5-speed with the connector just behind the left rear seat, in the spare tire area. One of the wires goes to the differential for the speed sensor. Re-seat or clean the connections and see if that fixes it.
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Oh wow! Paint on the rubber grommet around the trunk release and the between-panel gasketing on the bumper. It is a 15-footer, but I agree, the 928 is beautiful in yellow. Do you know if that is enamel or is there clear over the top? Are there runs in the paint, or just poor surface quality?. Some compound and a buffer and a crapload of time will fix those issues. You might even be able to replace the rubber seals (if you are very careful and lucky) without needing to respray. Of course someone here will say that you need to pull the whole thing apart and redo it frame-up, but I'm guessing that touch-up paint + TLC will considerably "shorten" that paint job distance. GL and great find!