Low RPM cutout or misfire in 951. *Video* - Rennlist - Porsche Discussion Forums

Notices

Low RPM cutout or misfire in 951. *Video*

 
Old 07-30-2007, 08:27 PM
  #1  
flosho
User
Thread Starter
 
flosho's Avatar
 
Join Date: Sep 2004
Location: Eau Claire, WI
Posts: 606
Default Low RPM cutout or misfire in 951. *Video*

My buddy's 86 951 is having some problems.

Basically it has a problem where it wont rev past 2,000 RPM. It just sputters and misses at 2k rpm and will stumble back down to idle.

I've attached an in car video, you can see it slowly rev to 2k and then bounce back down. During this video im not making the tach move like that, i'm just slowly rolling into the throttle to rev it up. At like 20 seconds in to the video, i have my foot about 1/2 in the throttle and it bounces back down.. this is exactly what is happening.


Sometimes it does, sometimes it revs fine!

http://www.youtube.com/watch?v=UI0clFZvbX0
flosho is offline  
Old 07-30-2007, 08:37 PM
  #2  
adrial
Super User
 
adrial's Avatar
 
Join Date: Jun 2001
Location: Northern NJ
Posts: 7,426
Default

I would guess some sort of wiring/electrical issue and would pay particular attention to the AFM. While it is having the problem, put a multimeter on the power going to the AFM (just pull back the boot and poke the probe in the back of the connector going into the AFM) and see if it is steady.

You can also check the resistance of the AFM itself by manually operating the barn door and measuring the resistance across it. The procedure has been posted here on the forums at some point and is also in the factory service manuals.

The crank sensors are also a good bet...both the wiring to them and the sensors themself. Paragon has relatively cheap replacements.
adrial is offline  
Old 07-30-2007, 10:26 PM
  #3  
hosrom_951
UAE Rennlist Ambassador
Rennlist Member
 
hosrom_951's Avatar
 
Join Date: Jan 2004
Location: UAE & Germany
Posts: 9,142
Default

Looking at the boost gauge while reving up, i would say firstly check for vacuum leaks, it appears that you got a serious one after the AFM/MAF/MAP (whatever you are running).

You should not be able to hit 1 bar on the gauge (0psi or 1 atmosphere) at such low revs.

Pressure test the system, start at 10psi, then push it to 20psi and listen for noises, and use soapy water to spray on suspected area.

Once that is done, and you are sure that it is leak free (vacuum wise) checking the electrical components would be fun.
hosrom_951 is offline  
Old 07-30-2007, 11:23 PM
  #4  
Tom M'Guinn
Addict
Rennlist Member

 
Tom M'Guinn's Avatar
 
Join Date: Jan 2002
Location: Just CA Now :)
Posts: 10,637
Default

Have you checked the TPS switch? It tells the motor to shut off the injectors when the throttle is shut above about 1600 rpms (1650?).
Tom M'Guinn is offline  
Old 07-31-2007, 01:03 AM
  #5  
goody944t
New User
 
goody944t's Avatar
 
Join Date: Jun 2007
Location: Minneapolis, MN/ Eau Claire, WI
Posts: 4
Default

which suggestion would be best to look into first? if it is the tps switch why would it rev up over 1600rpms some of the time? this is for my car by the way...haha
goody944t is offline  
Old 07-31-2007, 03:16 AM
  #6  
Kell
User
 
Kell's Avatar
 
Join Date: Nov 2005
Location: Winston-Salem, NC
Posts: 118
Default

My car did this exact same thing when I first started it after my rebuild. It would idle but then cough, sputter, and miss like crazy, hardly making it to 3k. I tried everything to fix it, the whole list. Finally i pulled the speed and reference sensors and it ended up being the set screw in the flywheel that the reference sensor reads off of. It hadnt been tightened down properly and it was just loose in the hole so it wobbled around. I replaced it with another one, tightened it down good, and re adjusted the speed/reference sensor heaight and it ran like a champ.

Check the speed and reference sensors too see if they're in good condition and check the height. Also check the set screw, youll have to use a flashlight witht he ref. sensor out and rotate the crank untill you find it, check and see if its set in all the way.

hope this helps, I know how fustrating it can be,
Good luck,
Kell
Kell is offline  
Old 07-31-2007, 04:45 AM
  #7  
Keithr726
Super User
 
Keithr726's Avatar
 
Join Date: Sep 2005
Location: Napa Valley and Berkeley
Posts: 5,105
Default

What does the car have mod wise?
Keithr726 is offline  
Old 07-31-2007, 03:28 PM
  #8  
flosho
User
Thread Starter
 
flosho's Avatar
 
Join Date: Sep 2004
Location: Eau Claire, WI
Posts: 606
Default

Pretty much stock, no muffler though, lowered.
flosho is offline  
Old 07-31-2007, 03:31 PM
  #9  
xsboost90
Now Cool
Rennlist Member
 
xsboost90's Avatar
 
Join Date: Jan 2003
Location: cincinnati,OH
Posts: 15,215
Default

rocks car did this- it would run, but not enough to drive it. AFM was stuck in one postion- dirt in there from lack of a filter made the flapper door stick.
xsboost90 is offline  
Old 04-20-2008, 07:34 PM
  #10  
flosho
User
Thread Starter
 
flosho's Avatar
 
Join Date: Sep 2004
Location: Eau Claire, WI
Posts: 606
Default

Still have this problem, keep in mind this issue is intermittent, and is still pretty drivable, it just happens that sometimes it stops revving at just below 2,000 RPM, but then you can rev it again and it revs fine!?
flosho is offline  
Old 04-20-2008, 07:39 PM
  #11  
Tom M'Guinn
Addict
Rennlist Member

 
Tom M'Guinn's Avatar
 
Join Date: Jan 2002
Location: Just CA Now :)
Posts: 10,637
Default

Did you ever change the TPS? If not, that's still my vote.
Tom M'Guinn is offline  
Old 04-20-2008, 10:58 PM
  #12  
flosho
User
Thread Starter
 
flosho's Avatar
 
Join Date: Sep 2004
Location: Eau Claire, WI
Posts: 606
Default

Ouch, $100 for a TPS, anyone selling a good used one?
flosho is offline  
Old 04-20-2008, 11:15 PM
  #13  
Ski
Addict
Rennlist Member

 
Ski's Avatar
 
Join Date: Nov 2001
Location: Heber Springs, AR
Posts: 7,892
Default

TPS, Speed and Reference sensor corrosion, or corroded grounds.

this can also happen, as mentioned, AFM. Over time, temp sensor can go, the wiper sweep inside can start wearing through and not providing the proper resistance that the DME reads for fuel.

Here is a TPS check page
http://www.arnnworx.com/tps-repair.htm
Ski is offline  
 


Thread Tools
Search this Thread

Contact Us About Us Advertising Cookie Policy Privacy Statement Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: