924Board.org

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Hi

I have a 1985 924 2.0 na

I have a problem, that as soon as I get to about 4000 rpm with full throttle, its almost like someone has hit the brakes... the engine just suddenly looses power.. but if you back off the throttle, it will get through this barrier and continue to accellerate..

It really is quite sudden, and quite a hard loss of power, almost like you hae hit a rev limiter... but it only does it with full throttle

The other odd thing, if you accellorate without full throttle (say 3/4 throttle) it will accellorate fine... and even at full throttle, all the way to 4000 rpm it accelorates just fine.

All I can think of, is maybe the fuel pump isnt flowing enough fuel, or that the in the tank fuel pump isnt flowing enough fuel, so when it hits a set amount of required fuel (as needed at 4000 rpm with full throttle) it is loosing fuel pressure..

Anyone seen this before?

I also have a hunch about the fuel metering head perhaps has a fault, or maybe the ignition system isnt delivering enough of a powerfull spark.. but I think these are more likely to be long shots...

Apart from this, the car starts fine, runs fine and there does not seem to be any problems.

I have so far changed the plug leads, the distributor cap, the rotor arm, the fuel filter and given it a tune up (set the mixture on a gas analyser and set the timing with a strobe timing light).

Apart from this, I am at a loss...

One question though, My 944 has no in the tank pump.. even though it seems to have the same kind of fuel pump, and the same kind of tank... why does the 924 need an in the tank pump, but the 944 does not?

I have a spare strainer-take off, for a 944, that goes where the 924 in the tank pump goes... what would happen if I were to remove the in the tank pump and fit the item that the 944 has in its place?

I was told by one mechanic, that the in the tank pump was only fitted there for use in hot climates, to make sure that the low pressure fuel in the line between the tank and the pump didnt boil in super hot climates when the car is stuck in traffic, due to its low pressure, but with the in the tank pump, it increases the pressure and so the boiling point.

So in other words, would deleting the in the tank pump be a good idea?

For now my problem is the power loss at 4000 with full throttle, so if anyone has any ideas, let me know!

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Not sure on the 924 but the word from Porsche on the 928's was that cars destined for hot climates got the intank pump. On my 931 I replaced the intank pump with the 928/944 strainer but at the same time I installed a Pierberg high volume (6.5 bar) external pump. It does sound like the internal pump is toast and is causing a restriction. Another possibility is that the cat (if installed) is partially plugged.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

rennbod924 · Joined: 23 Jan 2005 Posts: 21

No cat on this one.

Although the interesting thing, for a while after I had the exhaust changed, the symptoms seemed to get better for a while.

So anyone out there able to offer other ideas apart from the in the tank pump?? (I am going to try changing that this afternoon)

When it happens, its almost like someone has hit the brakes.. no real splutters or pops, just complete and sudden loss of power.. A friend said that when it happened, he thought he saw a puf of smoke from the exhaust when he was following me... but he was not sure..

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

If the smoke was black you probably have an ignition issue. May want to check the timing and see if the distributor is going too advanced at WOT.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

rennbod924 · Joined: 23 Jan 2005 Posts: 21

I know how to check the ignition timing (just about), but how do I check how far its getting advanced at WOT?

In fact, I think I need to brush up on my ignition knowledge, because I am now all confused! Does the vacuum advance unit advance get hooked up to the engine side of the throttle body, so when the throttle is closed it gets a vacuum and advances the ignition?

or is the vacuum advance hooked up on the other side of the throttle and advances the ignition when the throttle opens and causes a pressure drop on the other side of the throttle body.

All confused now..

I have checked the diaphram on the vacuum advance unit, and it seems to be working fine... I hooked up a mighty vac vacuum pump onto it and pumped it up and the diaphram held a vacuum.

I have also just put a stephascope on the in the tank fuel pump and listened to it... a very quiet hum.. then I disconnected power to it, and it still hums... so I gues the humming is the sound of the other fuel pump working.

I have a fuel pressure guage, with some adapters to hook up to CIS injection... Where do I plumb them in and what pressure should I see?

At the moment I have the tank draining... ready to pull the in the tank pump, that should be done shortly and a 944 strainer put in its place, unless someone here comes along and tells me thats a bad idea in the meantime.

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

The vacuum advance should be hooked up to the filter side of the throttlebody so it only advances when the throttle is open. There should also be a centrfugal advance inside the distributor which may have a broken or weak spring.

The CIS gauge is hooked up between the center line on the fuel distributor and the WUR (control pressure regulator). When you hook it up make sure the valve is on the WUR side of the gauge.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Hmm... just set the timing to about 8 degrees before top dead centre with the vacuum hose disconnected from the advance.

Is that about right for a euro 924?

Removed the in the tank pump, which didnt seem to be running on the car... and replaced it with a 944 strainer.

Gave it a good hard rev, and I seem to have a bit of black smoke out the back when revved up hard... so I will have to get the mixture set on it right. (about 2.0% CO, is that right???)

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Not sure about the timing on your euro as mine Hayne doesn't cover that year. Is there an underhood label. If you have a CO gauge you want it to run just on the rich side of stroimetric(sp?) through the power band. A technique that is used here is to find the center point between the rough running lean and rich adjustments then richen it a few degrees.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Just as an update...

Just taken the car out, its running better and the power seems to be up, but at 4000 rpm it still bogs down.

It does not always seem to do it, sometimes it seems ok, but most of the time it will bog down.. it seems worse when hot, but I am not sure

One thing I did notice on the test run though, normally when it bogs down my instant reaction is to let off the throttle a bit and it will accellorate again. But tonight, I thought I would keep the throttle down to the floor, and it seems that if you keep the throttle down the rpm will continue to rise slowly and once the revs are at about 4500 to 5000 it will pick up again as if nothing was wrong..

So.. it seems to be speed related to between 4000 and 4500 where it just looses all power.. really odd.

Paul · Posted: Mon Jan 24, 2005 4:59 pm Post subject:

Pull the fuel pump relay and jumper it as per the Haynes manual, then take your car it for a ride.

If it's better, replace the relay.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Paul,

Thats for that, I will try it out.

Which pins do I need to jumper together? is it like the 944 DME relay where you jumper the 3 big pins together?

I will try it tonight if I can figure out which pins to jumper.

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Oh... I forgot to say..

Paul, have you seen a faulty relay acting like this before?

Paul · Posted: Mon Jan 24, 2005 10:52 pm Post subject:

I always look at the diagram on the relay to determine which pins to jumper. You jumper the switched contacts, not the field coil.

Your relay may have a faulty rev limiter (or other problem), and it only takes a couple of minutes to check.

Can't say that I have seen one cause your problem.

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Thanks for the FAQ CMXXXI, interesting reading.

Paul, The relay has a rev limiter built into it???

One question for everyone.... with the distributor cap off, if I turn the rotor arm by hand clockwise, it will turn about 5 degrees against springing force... I take it this is the vacuum or speed related advance mechanism.. but if I give it just a little more force, it will turn another couple of degrees and the rotor shaft lifts by a fraction of an inch!!!... as you release the clockwise twisting force, it will drop back down as it turns 2 degrees back, and stay at the same height as it returns to its rested position which is another 5 degrees anticlockwise.

Once I noticed this, I thought I would pull on the rotor arm shaft and it can be lifted up by about a quarter of an inch..

Is this normal? or could this be the problem (the rotor arm lifting at about 4000 rpm (engine speed related advance) with the throttle wide open (vacuum advance)

HELP!!!