1979 Porsche 928 Hesitation Problem

[zion76]

Hello all. I'm pretty new and finally picked up a 1979 5 Speed US 928. Timing belt/water pump was replaced as well as plugs,cap,rotor, fuel pump, fuel lines,cleaned fuel tank .. Problem is now while driving the car hesitates if you accelerate hard.. If you ease on it it drives regular.. Anyone know what possibly could be the problem? I checked for loose/broken vacuum lines already. thanks in advance for all who reply..

[MPDano]

Originally Posted by zion76

Hello all. I'm pretty new and finally picked up a 1979 5 Speed US 928. Timing belt/water pump was replaced as well as plugs,cap,rotor, fuel pump, fuel lines,cleaned fuel tank .. Problem is now while driving the car hesitates if you accelerate hard.. If you ease on it it drives regular.. Anyone know what possibly could be the problem? I checked for loose/broken vacuum lines already. thanks in advance for all who reply..

How about your Diverter Valve? Or Decel Valve? Mine was a different scenario but I did have a similar symptom. I also post this in the Common Fixes Thread.

[Guest765]

I hope you didn't buy the red one listed here in PA, it was clean but the asking price was about two grand too high and he wouldn't budge - been listed since last year.

[zion76]

Originally Posted by MPDano

How about your Diverter Valve? Or Decel Valve? Mine was a different scenario but I did have a similar symptom. I also post this in the Common Fixes Thread.

hi, and thanks for the quick reply.. i looked in the common problems thread can you point me in the direction of where the 2 valves that you mentioned are? i cant really see in the pics you provided.

[zion76]

Originally Posted by fwayfarer

I hope you didn't buy the red one listed here in PA, it was clean but the asking price was about two grand too high and he wouldn't budge - been listed since last year.

no it wasnt red.. mine is black on black 5 speed with 90k miles.

[MPDano]

Here's is the diverter valve





Decel is located on the side of your throttle body. Passenger side I think.

[zion76]

Originally Posted by MPDano

Here's is the diverter valve





Decel is located on the side of your throttle body. Passenger side I think.

thanks for the pic! wow i see this valve but i have a hose thats disconnected from it..ill try to see where it plugs into

[Danglerb]

The diverter valve sends air from the air pump during cold start rich running to the cats, otherwise it vent to the air filter box. I don't see it having any effect on running unless it isn't working the cats are fouling.

Deaccelleration valve bypasses the throttle to limit manifold vacuum during things like a down shift with high rpm and foot off the gas, otherwise manifold pressure is too low causing misfires. It should have NO EFFECT other than very low manifold vacuum conditions, ie foot off the gas entirely.

Hesitation I am thinking air fuel mix, or timing, but I'm not a CI$ guy.

[MPDano]

Both have hugeass diaphragms and known to cause hugeass vac leaks. Big vac leaks fo sho cause hesitation on throttle up. Especially since the Diverter is controlled by throttle vacuum. Like I said, mine was a different scenario (Euro CIS on a US Engine, specifically US Heads) that caused my issue, but the Diverter was a big part of that issue. Mine is actually blocked now and zero air goes to the Cat and I can still pass Calif Smog. With the help of a better Cat made by Magnaflow.

[zion76]

Originally Posted by Danglerb

The diverter valve sends air from the air pump during cold start rich running to the cats, otherwise it vent to the air filter box. I don't see it having any effect on running unless it isn't working the cats are fouling.

Deaccelleration valve bypasses the throttle to limit manifold vacuum during things like a down shift with high rpm and foot off the gas, otherwise manifold pressure is too low causing misfires. It should have NO EFFECT other than very low manifold vacuum conditions, ie foot off the gas entirely.

Hesitation I am thinking air fuel mix, or timing, but I'm not a CI$ guy.

thanks for the response!!!hmmm. i wonder if i should just plug them because the airpump has been disconnected as well on my car..

[zion76]

Originally Posted by MPDano

Both have hugeass diaphragms and known to cause hugeass vac leaks. Big vac leaks fo sho cause hesitation on throttle up. Especially since the Diverter is controlled by throttle vacuum. Like I said, mine was a different scenario (Euro CIS on a US Engine, specifically US Heads) that caused my issue, but the Diverter was a big part of that issue. Mine is actually blocked now and zero air goes to the Cat and I can still pass Calif Smog. With the help of a better Cat made by Magnaflow.

thanks for the replies! i also dont have the air pump functioning on my car. Belt removed.. Would it be hard to just block them off if they are not even being used?? If so would i just cap them off at both ends? thanks again for the wonderful advice!!!

[Kiln_Red]

Check your fuel pressure. Not sure what it should be for your MY and fuel setup. Considering the list of fuel parts that you have replaced, I would guess that the car had sat for a long period of time at one or more points in its life. It's possible that your FPR (fuel pressure regulator) and/or FPD (fuel pressure dampners) have seized diaphragms as I discovered on a poor running 928 this past weekend. Varnished fuel will do this. This will cause a lean or rich mixture at virtually any throttle position depending on what position the diaphragms seize. The diaphragms are also known to rupture. You can check the vac lines for the presence of fuel to confirm this.

I would think that your FPR and FPDs were in that bad of shape that your fuel distributor would be as well.

I'm not the most CIS savvy person out there.
Just a tip.

[zion76]

Originally Posted by Kiln_Red

Check your fuel pressure. Not sure what it should be for your MY and fuel setup. Considering the list of fuel parts that you have replaced, I would guess that the car had sat for a long period of time at one or more points in its life. It's possible that your FPR (fuel pressure regulator) and/or FPD (fuel pressure dampners) have seized diaphragms as I discovered on a poor running 928 this past weekend. Varnished fuel will do this. This will cause a lean or rich mixture at virtually any throttle position depending on what position the diaphragms seize. The diaphragms are also known to rupture. You can check the vac lines for the presence of fuel to confirm this.

I would think that your FPR and FPDs were in that bad of shape that your fuel distributor would be as well.

I'm not the most CIS savvy person out there.
Just a tip.

thanks for the info!!! ill check on this tommorow..

[DPW928]

The problem you are having could also be due to the vacuum lines to the distributor being reversed.

[MPDano]

Wouldn't that make it run bad at all ranges though? I dunno, just wondering.

[DPW928]

Originally Posted by MPDano

Wouldn't that make it run bad at all ranges though? I dunno, just wondering.

It would still have retarded timing but not as much as WOT. Also, the engine needs more advance to accelerate than it does to cruise.

[MPDano]

Originally Posted by DPW928

It would still have retarded timing but not as much as WOT. Also, the engine needs more advance to accelerate than it does to cruise.

Good to know.

[zion76]

***UPDATE***

So this morning i go out to start the car and i get the usual hard starting.. It actually starts on the 3rd attempt and would go through the warm up cycle.. i timed it BEFORE to take about 5 min before the high idle came down to 800..today was very different though.. The high idle stayed high and WOULD not come down even though the car is warm which is so leading me to think its the WUR thats faulty.. Previous owner told me that they had cleaned it out but it NEVER was replaced.. now im researching how to test the WUR to see if its defective.

[MPDano]

Originally Posted by zion76

***UPDATE***

So this morning i go out to start the car and i get the usual hard starting.. It actually starts on the 3rd attempt and would go through the warm up cycle.. i timed it BEFORE to take about 5 min before the high idle came down to 800..today was very different though.. The high idle stayed high and WOULD not come down even though the car is warm which is so leading me to think its the WUR thats faulty.. Previous owner told me that they had cleaned it out but it NEVER was replaced.. now im researching how to test the WUR to see if its defective.

You'll need CIS Fuel Pressure gauges to do this, which a must have tool with any CIS car.

[jackforey]

Bro thats a nice car whats the last four of your vin? and how is the project comming?