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-   -   engine can't be revved up (https://www.alfabb.com/bb/forums/spider-105-115-series-1966-1994/587586-engine-cant-revved-up.html)

mcola 05-22-2017 03:24 PM

engine can't be revved up
 
Helping a fellow Alfisti with his 86 Veloce.
He drove it somewhere, parked it and then couldn't get it to restart and hour later.
It had spark, so we started chasing fuel.
Fuel relay, inline fuse, flywheel sensors, battery voltage - all good.
I became convinced it had to be the fuel pressure regulator, so I started to pinch the return line and was able to get it to start and run for a few seconds. I gave him an old spare from a parts car, it got installed and now the car starts and idles, however the engine can't be revved much (maybe to 1500 or so) without getting rough and dying.

I can hear and feel the main pump running, I can hear and feel the in-tank one running, and we pulled it for good measure, and the tank is clean and the stepped hose is intact. Not sure if the sock filter was attached as they tend to fall off as you pull the intank pump. The fuel filter was replaced last year.

I cannot find any vacuum leaks.

At this point, I have a question... can a fuel pressure regulator fail in such a way that it can build up the necessary pressure at idle and not when there is a higher demand for fuel? I tried pinching the return line a bit but it didn't seem to make much difference.

Thanks

CanHam86Spider 05-22-2017 04:19 PM

Thanks for starting this thread Mike! Maybe another thing worth mentioning is that I can hear the main fuel pump working when idle. When it stalls out you then hear the pump turn off immediately after it dies. Doesn't sound like it shuts off before it dies.

John533i 05-22-2017 06:55 PM

Your main pump may be on it's last legs. A fuel pressure gauge isn't very expensive on Amazon; if you install it inline with the cold start injector, you can determine if you have the correct pressure at idle and under throttle, plus if it bleeds off too quickly.

mcola 05-23-2017 06:30 AM

That's a good idea. Thanks

mmarvi 05-23-2017 07:23 AM

Could it something else? A failed Cat can mimic the same problem.

mcola 05-23-2017 07:39 AM

Quote:

Originally Posted by mmarvi (Post 7631522)
Could it something else? A failed Cat can mimic the same problem.

I did wonder that myself but kinda ruled it out for now because there was no evident slow deterioration of the exhaust. No funny rattles or noticeable gradual loss of power. Car ran fine, turned it off and sudden problem!

I know there is a vacuum pressure test that can be used to determine possible exhaust blockages but can barely get the engine off idle so might not be possible to test.

mmarvi 05-23-2017 07:49 AM

It's easy to test with a hacksaw, and a length of replacement pipe!:smile2:

Quote:

Originally Posted by mcola (Post 7631546)
I did wonder that myself but kinda ruled it out for now because there was no evident slow deterioration of the exhaust. No funny rattles or noticeable gradual loss of power. Car ran fine, turned it off and sudden problem!

I know there is a vacuum pressure test that can be used to determine possible exhaust blockages but can barely get the engine off idle so might not be possible to test.


ghnl 05-23-2017 08:54 AM

Common things happen commonly. So check for the usual suspects first.

Is the wire harness connected to the Air Flow Meter? If so, peek inside the connector to be sure the metal connectors have not become displaced. Check the air ducts across the top of the engine. Splits in the accordion section are common - it can be closed while idling but open up when rev'd as the engine rocks on its mounts. Is there enough gas in the tank? If the in-tank pump is weak or not functioning it'll have problems running when the fuel level is below 1/2 full. If possible, add 5 gallons. Did you fill up just before stopping? Could that gas be contaminated? Check inside the vacuum hose that goes to the fuel pressure regulator. There should be no evidence of raw fuel in that hose. If there is the FPR has failed.

mcola 05-23-2017 11:12 AM

Quote:

Originally Posted by mmarvi (Post 7631562)
It's easy to test with a hacksaw, and a length of replacement pipe!:smile2:

I was thinking of splitting the downpipes from the manifold or pulling the O2 seeing as they are more reversible if the cat isn't the problem.

mcola 05-23-2017 11:18 AM

Quote:

Originally Posted by ghnl (Post 7631642)
Common things happen commonly. So check for the usual suspects first.

Is the wire harness connected to the Air Flow Meter? If so, peek inside the connector to be sure the metal connectors have not become displaced. Check the air ducts across the top of the engine. Splits in the accordion section are common - it can be closed while idling but open up when rev'd as the engine rocks on its mounts. Is there enough gas in the tank? If the in-tank pump is weak or not functioning it'll have problems running when the fuel level is below 1/2 full. If possible, add 5 gallons. Did you fill up just before stopping? Could that gas be contaminated? Check inside the vacuum hose that goes to the fuel pressure regulator. There should be no evidence of raw fuel in that hose. If there is the FPR has failed.


I did check to make sure the AFM was plugged in but never thought to check the pins. Worth a shot!
Did check the air tubes paying special attention to the accordian and they are good.
Should have mentioned the tank was full, and he did fill up the day before (put about 50 miles on that tank)??
Should have also mentioned that before farting around with the FPR return line, I did pull off the vacuum and no fuel. Is that the only sign of a bad FPR? Can it produce the right pressure at idle and not at higher demand?

ghnl 05-23-2017 11:37 AM

2 Attachment(s)
The FPR is usually quite reliable. The most common failure is the internal diaphragm breaks allowing fuel to get to the (supposed to be) closed vacuum side.

The hose attached to the Cold Start Injector uses a clamp (unlike at the fuel rail). So it is relatively easy to remove the hose there and tee in a gauge. Too much or too little fuel pressure will prevent the engine from running properly.

spiderserie4 05-23-2017 11:53 AM

great photo of the FPR internals, Eric!
@Mike, did you test the FPR vacuum line pulls vacuum? Is perhaps the nipple on the plenum blocked?

mcola 05-23-2017 12:57 PM

Quote:

Originally Posted by spiderserie4 (Post 7631858)
great photo of the FPR internals, Eric!
@Mike, did you test the FPR vacuum line pulls vacuum? Is perhaps the nipple on the plenum blocked?

I did, and it does pull vacuum.

mcola 05-23-2017 12:59 PM

Quote:

Originally Posted by ghnl (Post 7631850)
The FPR is usually quite reliable. The most common failure is the internal diaphragm breaks allowing fuel to get to the (supposed to be) closed vacuum side.

The hose attached to the Cold Start Injector uses a clamp (unlike at the fuel rail). So it is relatively easy to remove the hose there and tee in a gauge. Too much or too little fuel pressure will prevent the engine from running properly.

Yes, cool pic!
Does the fuel pressure gage actually have to be t'd in? Can it just be plugged into the end of the hose when its pulled off the csi, realizing the csi won't be able to spray, so perhaps doing it when the engine is warmed up?

GV27 05-23-2017 02:05 PM

Quote:

Originally Posted by mmarvi (Post 7631562)
It's easy to test with a hacksaw, and a length of replacement pipe!:smile2:

Hard to see that being easier than plugging in a vacuum gauge.


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