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Loosing power/boost after driving after an hour or more
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chuck21401  



Joined: 20 Feb 2005
Posts: 107
Location: Annapolis, MD

PostPosted: Tue Jun 11, 2019 1:25 am    Post subject: Loosing power/boost after driving after an hour or more Reply with quote

I have a 1981 931 with a 1980 engine/drivetrain. The runs strong but I've noticed a loss in power/boost after driving it for an hour or two. I can hear the turbo spinning up but after about 4000rpm the power drops off. It doesn't cut off or shut down...just seems to trail off. Shifting to the next gear (and lower RPM) the turbo spins up again and builds boost...but at around 4000rpms the power drops off again. I don't have a boost gauge yet. The temperature gauge seems normal. If I shut the car down for a couple of hours and then go for a drive, the power mysteriously comes back.

I've spent some time reading through threads on the forum (including the post "A Systematic Approach to 931 Diagnostics") and not entirely sure where to start since the problem is intermittent.

The previous owner rebuilt the engine, turbo and wastegate.

In related news, I will be looking for a shop in the MD metro area that is familiar with 924 turbos if anyone has recommendations.

Chuck
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bureau13  



Joined: 07 Sep 2017
Posts: 160
Location: South Florida

PostPosted: Tue Jun 11, 2019 2:00 am    Post subject: Reply with quote

Is it a fairly shop drop in power? Or more like it simply makes less power at that point than it did prior to this?

The boost gauge will help with diagnosing boost issues, even if you don't install it properly you can hook it up to a boost source and lay the gauge somewhere temporarily.
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-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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chuck21401  



Joined: 20 Feb 2005
Posts: 107
Location: Annapolis, MD

PostPosted: Tue Jun 11, 2019 2:46 am    Post subject: Reply with quote

bureau13 wrote:
Is it a fairly shop drop in power? Or more like it simply makes less power at that point than it did prior to this?

The boost gauge will help with diagnosing boost issues, even if you don't install it properly you can hook it up to a boost source and lay the gauge somewhere temporarily.


I would describe it as a plateau. The RPMs continue to build but weak from 4000rpm - 5000rpms. Thinking that something was wrong I didn't push it further.

For the first hour driving the car it pulls very strong to the redline in the first four gears (redline in 5th gear would get me in trouble).

Good point about the boost gauge. I'm going to order one this week; it will likely replace the clock in the console. I'm thinking about the boost/vacuum gauge from VDO. My next question will be asking about where to pick up the signal. To get a vacuum reading I believe that has to be after the throttle body?

Here's a pic of the engine bay. 1980 engine with S2 charge tube and throttle body.


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bureau13  



Joined: 07 Sep 2017
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Location: South Florida

PostPosted: Tue Jun 11, 2019 5:52 am    Post subject: Reply with quote

If you trace that vacuum line back from the vacuum advance pod on the distributor, where does it take you? Eventually it should lead to a vacuum/boost source after the throttle body, and you can T off of that.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1766
Location: Somerset, UK

PostPosted: Tue Jun 11, 2019 3:34 pm    Post subject: Reply with quote

Itís looks like a hybrid of series 1 distributor but with series 2 intake system?? Check your timing and ignition advance / retard. And all vacuum lines are correct and air tight. Sometimes heat can open up a leak not normally seen at lower temps.

Check the dizzy rotor is fully seated and not wobbling on the spindle. Iíve had issues of this previously causing low power at higher rpm. Will only use Beru or Bosch rotors now.

Low res pic so canít zoom in properly, but on the throttle body there is a yellow plug on a spigot. Use that spigot to pipe in a boost gauge post TB flap. This will tell you if you are making or leaking boost all the way through rpm

A tip. Your overboost switch has been earthed to the block so is as useful as a chocolate fire guard if the boost spikes beyond 1.0bar or so. It will grenade your motor.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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morghen  



Joined: 21 Jan 2005
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PostPosted: Tue Jun 11, 2019 6:18 pm    Post subject: Reply with quote

Replace the large o-ring between the S2 charge tube and the TB.
Also use some Victor Reinz or substitute permanent silicone that is oil resistent as well.
That o-ring will wear down into the charge tube and the o-ring itself gets soft by years of being in oil and maybe you're leaking boost there.

Mine did.
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chuck21401  



Joined: 20 Feb 2005
Posts: 107
Location: Annapolis, MD

PostPosted: Wed Jun 12, 2019 1:40 am    Post subject: Reply with quote

bureau13 wrote:
If you trace that vacuum line back from the vacuum advance pod on the distributor, where does it take you? Eventually it should lead to a vacuum/boost source after the throttle body, and you can T off of that.


Thanks.

That line leads to this port circled in yellow.

Carrera RSR suggests using the other port...I'm guessing that was something needed for the S2 but not needed for S1?


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chuck21401  



Joined: 20 Feb 2005
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Location: Annapolis, MD

PostPosted: Wed Jun 12, 2019 1:53 am    Post subject: Reply with quote

Carrera RSR wrote:
Itís looks like a hybrid of series 1 distributor but with series 2 intake system??


Yes. Previous owner installed Series 1 drive train into Series 2 car...but also installed series 2 throttle body and charge tube.

Quote:
Check your timing and ignition advance / retard. And all vacuum lines are correct and air tight. Sometimes heat can open up a leak not normally seen at lower temps.
Check the dizzy rotor is fully seated and not wobbling on the spindle. Iíve had issues of this previously causing low power at higher rpm. Will only use Beru or Bosch rotors now.


Thanks...will check those items.

Quote:

Low res pic so canít zoom in properly, but on the throttle body there is a yellow plug on a spigot. Use that spigot to pipe in a boost gauge post TB flap. This will tell you if you are making or leaking boost all the way through rpm


I appreciate that...posted another picture above.

Quote:
A tip. Your overboost switch has been earthed to the block so is as useful as a chocolate fire guard if the boost spikes beyond 1.0bar or so. It will grenade your motor.


Thanks for spotting that. I reconnected the overboost switch and will go for a drive after work. Of course I'm wondering why it was grounded in the first place.
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chuck21401  



Joined: 20 Feb 2005
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Location: Annapolis, MD

PostPosted: Wed Jun 12, 2019 10:14 am    Post subject: Reconnected the overboost switch Reply with quote

So I reconnected the overboost switch and went for a ride. I started off slowly until the car was warmed up.

I could hear the turbo spin up as usual and gave it more gas in second gear. Everything was fine until about 5000 rpm when the reconnected overboost switch abruptly shut down the fun. I drove normally and tried to build boost in 3rd gear -- same problem at around 5,000rpm.

I've ordered a boost gauge so hope to get a better idea about what is going on soon.
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Carrera RSR  



Joined: 08 Jan 2010
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Location: Somerset, UK

PostPosted: Wed Jun 12, 2019 5:56 pm    Post subject: Re: Reconnected the overboost switch Reply with quote

chuck21401 wrote:
So I reconnected the overboost switch and went for a ride. I started off slowly until the car was warmed up.

I could hear the turbo spin up as usual and gave it more gas in second gear. Everything was fine until about 5000 rpm when the reconnected overboost switch abruptly shut down the fun. I drove normally and tried to build boost in 3rd gear -- same problem at around 5,000rpm.

I've ordered a boost gauge so hope to get a better idea about what is going on soon.


Make sure the boost tube is well earthed. If not it can prematurely activate the boost switch cutting the fuel pump. Switches normally fail open meaning the fuel pump is permanently cut. Hence some folks bypass it rather than buy another. Letís hope itís not cutting out due to overboost!

A boost gauge will massively help you know if this issue is boost or fuel related. Did it cut out the same as before changing the switch?
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252
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chuck21401  



Joined: 20 Feb 2005
Posts: 107
Location: Annapolis, MD

PostPosted: Thu Jun 13, 2019 12:30 am    Post subject: Re: Reconnected the overboost switch Reply with quote

[quote="Carrera RSR"]
chuck21401 wrote:
Did it cut out the same as before changing the switch?


It's hard to say. I was expecting the overboot switch to cut off the fuel....so didn't floor it all of the way....wasn't sure how dramatic the effect would be.

Quote:
Make sure the boost tube is well earthed.


Is there something to look for on the parts diagram to verify that is connected in the car? Some sort of grounding strap?

My other idea is to trace the wire for the over boost switch back through the harness to see if it is frayed/cracked along the way.
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chuck21401  



Joined: 20 Feb 2005
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Location: Annapolis, MD

PostPosted: Fri Jun 14, 2019 11:49 pm    Post subject: Reply with quote

I bought a VDO vacuum/boost gauge on Ebay. This morning i connected it temporarily, taped it to the dashboard and went for a run. When I get on it in 2nd and 3rd gear I am seeing just under 14psi. Hmmm a bit more than stock boost.

I forgot to mention that the car has an AEM air fuel ratio gauge already installed. Under boost the gauge is reading in the 13s.
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Fasteddie313  



Joined: 29 Sep 2013
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PostPosted: Sat Jun 15, 2019 2:32 am    Post subject: Reply with quote

chuck21401 wrote:
I bought a VDO vacuum/boost gauge on Ebay. This morning i connected it temporarily, taped it to the dashboard and went for a run. When I get on it in 2nd and 3rd gear I am seeing just under 14psi. Hmmm a bit more than stock boost.

I forgot to mention that the car has an AEM air fuel ratio gauge already installed. Under boost the gauge is reading in the 13s.


13 is a little high. 14psi @ 13s AFRs and no intercooler sounds a bit dangerous..

If their is no boost controller on your WasteGate you might have a sticky WG..

Pull the WG line off your upper charge tube and hit that like with like 15-30psi of compresed air a bunch of times to open and close the WG and exercise it. That might loosen it up..

I would also set your AFR screw with the dwell meter method to about 40-45 on the scale which might get your AFRs down if that is the problem.. Also test your TB microswitches while you are doing that..
You didn't happen to set your AFR screw with your WBO2 gauge did you? That is not a good idea if your lambda system is running..

Other than that a full CIS test and test that your WUR is dropping control pressure when pressure is applied to its port..

Are all those little vacuum lines from your TB circled in yellow to your WUR and stuff new?

Does your WBO2 have any sort of calibration procedure?
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bureau13  



Joined: 07 Sep 2017
Posts: 160
Location: South Florida

PostPosted: Sat Jun 15, 2019 3:35 am    Post subject: Reply with quote

This is so weird...your car is the same color as mine, mine also had the overboost switch grounded and when I put a boost gauge on it, I also was pegging around 15 psi. Are you sure you're not testing my car???

I don't have an air/fuel meter though. Wish I did...

chuck21401 wrote:
I bought a VDO vacuum/boost gauge on Ebay. This morning i connected it temporarily, taped it to the dashboard and went for a run. When I get on it in 2nd and 3rd gear I am seeing just under 14psi. Hmmm a bit more than stock boost.

I forgot to mention that the car has an AEM air fuel ratio gauge already installed. Under boost the gauge is reading in the 13s.

_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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chuck21401  



Joined: 20 Feb 2005
Posts: 107
Location: Annapolis, MD

PostPosted: Tue Jun 18, 2019 11:08 am    Post subject: Reply with quote

bureau13 wrote:
This is so weird...your car is the same color as mine, mine also had the overboost switch grounded and when I put a boost gauge on it, I also was pegging around 15 psi. Are you sure you're not testing my car???

I don't have an air/fuel meter though. Wish I did...


It sounds like disconnecting the overboost switch is a risky temporary solution...but I have not tracked down the cause in my car. I have some other electrical gremlins that could be very related. Occassionally my windshield wipers will come on randomly....but then the problem goes away after running the car for awhile.

Also appears that my car is making more boost than it should...makes me wonder which spring is in the wastegate...not exactly easy to check right?
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Last edited by chuck21401 on Tue Jun 18, 2019 11:46 am; edited 1 time in total
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