924Board.org

[Levans]

hey all,
now it is summer here and over the last 3 months the weather temps have been pretty high. 30-36 deg C.

Heres the problem>

on a hot day after driving for about 10-20 mins, at idle the RPM needle will start to "flick"(like an electric spike). either up or down approx 300-400 RPM(mostly down).
if its a very hot day the engine will stutter when it does this RPM "flicking"
and sometimes on accelleration there is a miss, like a shortage of fuel. there is no noticable change in engine revs when the RPM gauge flickers unless it starts to stutter.
While this is happening at idle the fuel pump makes a shorting type of noise. i don't know if its turning on and off when it does this.

when the car is driven on a cooler day it will not do this at all and will idle propperly when warm and drive how it should.

When i initially bought my 78 924 it had a bad fuel pump which caused a bad (undrivable) hessitation about 5 mins after warm up and a high pitched noise, so i put in a brand new bosch fuel pump. solved that.


My Question>

1- Is my year and a half old fuel pump on its way out or is it
something else?

I know it is most likely a fuel problem due to symptoms.

wierd part>

1-after replacing the original fuel pump a year and a half ago i noticed a low idle at cold startup(low fuel pressure?), whereas the origional BAD pump had a good idle at cold startup.(from memory)

2- when driving at low RPM and engine is under load (incline, foot flat to floor) it will stutter and hessitate. and i know that its not how i am driving because my friends 924 doesn't do that at all.

are these symptoms of low fuel pressure?

Have i replaced the original fuel pump with a faulty one?
or even the wrong One?

I will list all the parts i have replaced to try and narrow it down
(and its a long list)

please can someone help me...
much appriciated
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78 Dark met Green "THE 924"

[OR_Sunset]

Okay... I have seen that particular problem only once, and it was electrical. I found that the lead on the neg terminal of the battery was loose, and it was vibrating while driving.

Check your grounds, engine ground strap, make sure spark plug wires aren't fouling or arcing anywhere, dist. cap, rotor, etc. The whole electrical works. It sounds like an intermittent spark misfire, IMHO.

However, to rule out fuel starvation:

I would recommend getting a decent quality fuel pressure gauge. Plumb it into the fuel feed line, and you can see if your fuel pump is pushing spec psi. Then connect a handheld pressure gauge to an injector line, and see if the right pressure is making it past the regulator. (Haynes has the specs)

Or you could do the injector flow test, with four 1L water bottles, a ruler, and a stopwatch.

If its the fuel system, my guess would be the control pressure regulator. These can sometimes gum up, and not let enough fuel through.

Hope that helps.
_________________
82 924 N/A -
9.3:1 CR Pistons, Dual Webers, Stage 2 Cam.
4-1 Headers, DynoMax exhaust, Monza tips.
MSD 6AL Ignition, 944racing short shift kit.
Through hood cold air intake with scoop, K&N.
Silver/Blue.

[OR_Sunset]

Oh, and I just re-read your post. The stuttering and hesitation under load could be a serious mystery, or it could be dead simple.

Check your timing. A little more initial advance might allow you to run lower RPMs under load. (better low end power)... If your timing is off to the retard side, that might explain your whole problem. The tip-off to me would be the low idle.

And for the sake of your car, downshift so you keep the rpms up... anything below 2500 rpms under load is going to cause some wierdness eventually.
_________________
82 924 N/A -
9.3:1 CR Pistons, Dual Webers, Stage 2 Cam.
4-1 Headers, DynoMax exhaust, Monza tips.
MSD 6AL Ignition, 944racing short shift kit.
Through hood cold air intake with scoop, K&N.
Silver/Blue.

[kike924]

I had a similar problem with my idle speed going up and down until the engine stopped.
Before that I was having a low gas mileage problem that my mechanic tried solving reducing the gas intake.
Now my car idles very well but my mileage is lousy again
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Euro 924 1980 N/A
Daily driver + week end racer

[Lizard]

I would start by bridging the fuel pump relay terminals #30 and #87 take it for a drive and see if it stutters, the fuel pump relay is controlled by spark no spark the relay cuts the fuel to prevent a fuel spill in an accident. now be aware by bridging the terminals #30 and #87 the fuel pump will always be on, now you can jumper #15 and #87 however the #15 wire is not of thick enough guage and could cause problems down the road, (dont ask) so if you use #15 dont use for more than about 10-15 sec,

I would recommend checking in this order after doing the fuel pump relay,
#1 the wire (2 pin) that plugs into the distributor housing for hall effect sensor, the wires break and will loose connection and cause this to happen,
#2 all the connections at the coil, just replace the connectors on here,
#3 replace the connectors on the starter for the 2 smal wires,
#4 run a new wire (make sure you have a heat shield on it) to the large terminal of the starter,
#5 run a new wire (min 4 guage) from the large terminal of the alt to the positive term of battery,
#6 run a large (min 2 guage) wire from the mount for the ground of the battery to a solid point on the block (starter bolt??)
#7 pull the plug to the ign control unit (infront of left wheel in fender) and make sure all the connectors are good and not corroded, also follow the brown wire out of here and make sure it is getting a good ground
#8 up behind (well in front really when you think about it) the fuse box are 2 crown ground points, replace them and all the connectors on them a distrobution block works awsome for this.

car should be running fine now, if not then let us know and we will give you further instructions,
all these steps are of course after a basic tune up
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3 928s,

[Joes924Racer]

Ive had the same thing happen except mine quite
what I did was skip what Lizard said as I already
had done that & installed a performance electrical
high amp switch. I ran wires from the battery to
the pump to see if it work first. It worked better so
started checking things and found that the connection
at the pump had a fault and would break contact.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.