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Red face What are the symptoms of a bad fuel pump?

My '74 2.7S pulls strongly until it reaches 4000rpm, then it starts to stutter. (The best word I can think of, I'm not English speaking). It is a sudden thing, and not very subtle. The car literally shakes as the engine gets and loses power a few times per second. It can rev higher, but the effect is exactly the same at 4000rpm or 5000rpm. The noise is just more at 5000rpm.

Two days ago the car was still fine, the stuttering only started this morning. I could feel a temporary loss of power at about 5000rpm yesterday, but only for a fraction of a second. Then it went away and I never worried about it again.

Come to think of it - yesterday I was thinking: my fuel pump which is normally very loud, has been suspiciously quiet the last two or three days.

I don't think there is an electrical problem, I checked all the connections and replaced the points and condensor (no CDS).

Is this the typical symptoms of a fuel pump that's gone bad? Or can a very blocked fuel filter have the same effect?

Just making sure before I start ripping things out & replacing them.
Old 03-08-2002, 07:04 AM
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Patronus,

Your fuel pump output volume should be tested straight out of the pump ... no filter in the line, into a large glass container. The expected volume and timed duration is dependent on the model of the pump. The current drain of the pump during normal operation, i.e., hooked up and plumbed normally, should also be tested. No pump I am aware of should draw more than 8 Amps under load. Also, a distinct increase in the noise level, accompanied by either of the above 'out of spec' tests would be a sign of a defective pump needing replacement.

If the gas from the pump volume test has any visible rust in it ... the gas tank may need to be cleaned out and sealed. Be sure to remove the brass screen in the tank before applying sealant.
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Old 03-08-2002, 07:43 AM
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Early_S_Man, thanks. I'll try to find out how much fuel the pump should displace per second, and see if I can test it. Anybody on this board who can help me out with figures?

But first, something I forgot to mention: When I press the accellerator gently and let the engine gradually rev to 5000rpm (while driving) there's nothing wrong, it does so smoothly. But when I accellerate even moderately it stutters above 4000rpm. The deeper I press the accellerator, the bigger the problem seems to be. In other words, the problem only occurs when the engine is loaded, and the more loaded it is, the worse it becomes.

It is not the fuel filter, I made sure of that.
Old 03-09-2002, 06:00 AM
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Patronus,

I seem to be experiencing the same symptoms you described at the beginning of this thread...

At right about 4500 RPM, when I am really getting on the throttle, my car stutters, almost violently. If I accelerate slowly, or even moderately aggressively, there is no problems, no hesitation. Only when I am really pressing the accelerator.

Did you ever get this figured out?

Cheers!

Randy
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Old 08-08-2004, 06:31 PM
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Coil not being able to keep up after certain RPMs?
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Old 08-09-2004, 01:53 PM
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Check the ignition advane on the distributor......but if that is ok it may be the pump. If it is audibly louder than normal and noticing the sudden change in operating conditions.....it never did this before.....then it could be the pump starting to go. How old is it?
My 1979 911SC CIS pump failed last summer, and I noticed difficulty starting it one day which led up to increased occurences of hesitation while driving. Then one day it got loud while running and within 10 miles the car started shutting down. Let it cool for 10 minutes and it would run for a few blocks and shut down again. The pump was gone.
Order a new one if you suspect it is over 15 years old if the tests above don't show anything out of spec.....what have you got to loose?
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Old 08-09-2004, 02:27 PM
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Hmmmm, this is getting interesting.

Did a bit more driving today than usual and did some experimenting while I was out.

First gear, if I press the accelerator all the way to the floor, would stutter pretty badly, right around 4500 RPM, and through to 5500/6000.

Second gear, floored, would not duplicate...it accelerated to 6000 RPM with no troubles.

First Gear, when accelerated aggressively, but NOT FLOORED, worked fine, no stuttering.

Second gear, again...no problems.

Since I wasn't on any freeways today, I couldn't take any hire gears all the way to 6000 RPM, but the driving I did show no indications of any problems.

Basically, floored first gear causes stuttering...kinda violent stuttering....and all other gears and degrees of acceleration had no perceived problems.

Fuel pump going bad? Distributor vacuum advance? How about a dirty air filter? Could that be the culprit? I'm probably due a clean one...the current (K&N) has a little over 4000 miles on it.

Randy
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Old 08-09-2004, 06:00 PM
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Hi there.

I wonder if anyone could tell me what became of this. I'm having *exactly* the same problem with my 79 SC motor. I've totally ruled out the ignition system. Did it turn out to be a bad fuel pump?
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Old 08-22-2004, 06:21 PM
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If you want to rule out the distributor you should take readings of the advance curve at varying rpm's and plot a graph of advance versus rpm. Then compare this to the curve from the factory manual. This way You can see if the dual advance feature is working properly. Also, would not hurt to lube the distributor shaft while you are at it to keep the advanced weights moving freely.....this is a problem with sticking advance counterweights on these old engines.
I doubt an air cleaner would cause this as it would have to badly blocked and would affect all rpm range.
Do you have any vacuum leaks anywhere? This will cause problems too.
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Old 08-23-2004, 02:34 PM
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Old 08-23-2004, 02:34 PM
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