Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 911 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
Join Date: Aug 2008
Location: Lake Tapps, WA
Posts: 121
Garage
Possible issues with sensors/DME...need help

I have an 86 w/ stock 3.2 and B&B muffler. Car was running great, but was driving home the other day and the car just died. Turned over fine...just didn't want to start. We swapped the fuel pump, DME relay. No go. Then swapped out the DME computer from a car that was running fine (another 86 3.2 motor w/ chip). Started and ran rough, but thought it may have just been the chip.
Then...back to the same original problem after trying to start it again. Wouldn't start.

We swapped out the two sensors (Cylinder Head and RPM) from the other motor. Also looked at the flywheel sensor and it looked like it was in pretty good shape. The car started but ran rough. But back in the original DME computer and started it up. Still runs pretty rough and quite rich. Backfires a little and idles terrible.

Any thoughts or ideas would be much appreciated!!!
Old 09-16-2008, 08:49 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered User
 
cnielsen's Avatar
 
Join Date: Sep 2002
Location: Lake Tapps, WA
Posts: 2,970
I think Bernie said the car was running lean.

Anyway, parts ruled out are the DME, DME relay, fuel pump, and the two sensors (RPM and TDC).

It almost sounds like two different problems...one caused by the other.

Now the car starts every time but runs poorly, takes throttle but spits and pops, smells like spent fireworks.

Could the initial problem of a no-start and several attempts to restart cause another problem?

We are about the swap out the Air meter control unit but are open to ideas after that.
__________________
'67 911S
'69 911S,
'70 911ST
'73 911T Targa Signal Yellow
'78 911SC backdate
Old 09-16-2008, 09:25 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered User
 
db_cooper's Avatar
 
Join Date: Nov 2007
Location: Decatur, Ga.
Posts: 889
Garage
Could be the temp head sensor? Don't have the specs handy, easy enough to check with an Ohm meter. The "fireworks" odor could be a rich condition and the catalytic converter being overworked, if the sensor is open. Might be a loose connection to the sensor too. Worth a look.
Old 09-17-2008, 05:25 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Registered User
 
cnielsen's Avatar
 
Join Date: Sep 2002
Location: Lake Tapps, WA
Posts: 2,970
We checked that with an ohm meter and it appeared to be working...a little higher than spec but not too far off. Anyway, I think that is the first thing to look at...unplug it while the car is running and see if it runs any worse, if not, that may be the culprit.

Thanks for the idea!
__________________
'67 911S
'69 911S,
'70 911ST
'73 911T Targa Signal Yellow
'78 911SC backdate
Old 09-17-2008, 07:25 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
Join Date: May 2002
Location: St Louis
Posts: 4,213
can you put the questionable DME in the car that runs fine?
__________________
Rick
88 Cab
Old 09-17-2008, 07:48 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
Join Date: Feb 2004
Location: louisiana
Posts: 1,481
money on AFM
Old 09-17-2008, 07:52 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Registered User
 
cnielsen's Avatar
 
Join Date: Sep 2002
Location: Lake Tapps, WA
Posts: 2,970
I have not placed the suspect DME in the good running car yet as they are in different locations. I did confirm that the second DME continues to run fine as I reinstalled it in the good running car last night.

The CHT sensor and the AFM are the next step. Actually have a spare AFM that I can hook up to test. However, the question there is why would this instantly cause a stall, then a non-start, then a start but runs poorly issue. Seems like two separate issues that happened to arrise at the same time.

Hmmmm, carbs are sounding good about now!
__________________
'67 911S
'69 911S,
'70 911ST
'73 911T Targa Signal Yellow
'78 911SC backdate
Old 09-17-2008, 07:59 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
Join Date: Feb 2004
Location: louisiana
Posts: 1,481
time between switching or swapping parts could have an effect on the cht operation.
Could both cht's be bad?
A car could start and run for a while with a failing cht. And then fail to warm start.
Old 09-17-2008, 08:09 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Registered User
 
cnielsen's Avatar
 
Join Date: Sep 2002
Location: Lake Tapps, WA
Posts: 2,970
Could it cause a stall though at freeway speed? BTW, the car had been driven about 15 minutes when it stalled on the freeway.
__________________
'67 911S
'69 911S,
'70 911ST
'73 911T Targa Signal Yellow
'78 911SC backdate
Old 09-17-2008, 08:14 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Registered User
 
Join Date: Feb 2004
Location: louisiana
Posts: 1,481
yes it can, I've seen it happen. 15 minutes is probably about perfect time time to, depending on ambient temp.

but I'd swap afm's before I jacked up the car. Is there a cat?
Old 09-17-2008, 08:15 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Registered User
 
ischmitz's Avatar
 
Join Date: May 2002
Location: Santa Barbara, CA
Posts: 4,509
Garage
Send a message via Skype™ to ischmitz
Good call on swapping the suspect DME into a good running car to rule it out.

Next, a systematic check of sensors and actuators, etc:
- CHT is only suspect when running rought during cold engine. At temperature it should not have much of an effect
- O2 sensor
- AFM
- fuel pressure regulator
- leaking or stuck/clogged injector
- bad/stuck ICV

Cheers,
Ingo
__________________
'74 Targa 3.6 (not stock ) - '01 C4 (almost stock) - '00 ML430 (stock)

I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
Porsche "Hammer" or Porsche PST2 - I can help!!
How about a NoBadDays DualChip for 964 or '95 993

Last edited by ischmitz; 09-17-2008 at 08:36 AM.. Reason: re-reading the posts
Old 09-17-2008, 08:22 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Registered User
 
cnielsen's Avatar
 
Join Date: Sep 2002
Location: Lake Tapps, WA
Posts: 2,970
yep, original cat right now. Cat bypass is on order and supposed to be here on Thursday. New B&B muffler recently installed too.
__________________
'67 911S
'69 911S,
'70 911ST
'73 911T Targa Signal Yellow
'78 911SC backdate
Old 09-17-2008, 08:23 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #12 (permalink)
Registered User
 
Join Date: Feb 2004
Location: louisiana
Posts: 1,481
ischmitz knows plenty more than I do. But I experienced the opposite. Cold start fine, warm or hot start = fail.
Old 09-17-2008, 08:43 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)
Registered User
 
ischmitz's Avatar
 
Join Date: May 2002
Location: Santa Barbara, CA
Posts: 4,509
Garage
Send a message via Skype™ to ischmitz
Quote:
Originally Posted by jbrinkley View Post
ischmitz knows plenty more than I do. But I experienced the opposite. Cold start fine, warm or hot start = fail.
no no - I wouldn't say that. My observations of the CHT are from a 3.6 where it doesn't harm a warm engine. The DME simply ignores a missing or open CHT and assumes the engine to be at operating temperature. On the 3.2 what you have seen may well be true for the 3.2. So I would check the CHT. Measure resistance at room temperature to be around 2.2 - 2.8k Ohm.

Cheers,
Ingo
__________________
'74 Targa 3.6 (not stock ) - '01 C4 (almost stock) - '00 ML430 (stock)

I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
Porsche "Hammer" or Porsche PST2 - I can help!!
How about a NoBadDays DualChip for 964 or '95 993
Old 09-17-2008, 09:11 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
Registered User
 
cnielsen's Avatar
 
Join Date: Sep 2002
Location: Lake Tapps, WA
Posts: 2,970
OK, so a little update...

Got the cat bypass hooked up...wanted to do this anyway, but it made no difference in the poor running condition.

Swapped out the AFM from a known running car and it also made no difference.

Then, started the car and unplugged the CHT sensor while it was running. This made no difference, the car still hunted for idle and backfired. Soooo.... from deductive reasoning, it sounds like the CHT sensor is not doing what it is supposed to do.

A new CHT sensor is on order and hopefully that will put it back on the road!

Thanks for all your help and suggestions, it ultimately led us in the right direction.
__________________
'67 911S
'69 911S,
'70 911ST
'73 911T Targa Signal Yellow
'78 911SC backdate
Old 09-20-2008, 05:26 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #15 (permalink)
Registered User
 
ChrisBennet's Avatar
 
Join Date: Aug 2001
Location: Nashua, NH USA
Posts: 8,089
I would recommend against putting someone elses DME in the car being tested. A car can hurt a DME but a DME can't hurt a car. Always test your DME in a known good car - don't try/fry someone else's DME in your own car.

The cylinder head temperature (CHT) sensor on a 3.2 can certainly bring your car to a stop as it can make the engine go full rich. It will feel like it's bucking before then.

The CHT reads around 3600 ohms cold and 300 hot. A bad CHT or a bad ground with the earlier single wire (1 pin in the connector) looks like an open circuit (Infinite ohms which the computer reads as super super cold.) To compensate it makes the mixture super super rich. A meter won't always tell you if they are bad as they can be intermittent. I unplug the CHT and stick a 1/4 what resistor into the connector - say 3.3K ohms for a cold motor and 330 for a warm motor and take the CHT out of the equation that way.

-Chris
__________________
'80 911 Nogaro blue Phoenix!
'07 BMW 328i Coupe
http://members.rennlist.org/messinwith911s/
Old 09-20-2008, 10:01 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #16 (permalink)
Registered User
 
ischmitz's Avatar
 
Join Date: May 2002
Location: Santa Barbara, CA
Posts: 4,509
Garage
Send a message via Skype™ to ischmitz
Quote:
Originally Posted by ChrisBennet View Post
...but a DME can't hurt a car
Not quite right. A short in the injector final driver stage can damage the injectors in a good car. Once the peak-and hold current limiter is not working the low-impedance injectors can overheat and burn out. As a minimum you will flood the engine. Ask me how I know. Let's just say fuel was dripping out of the tail pipe....

On the other hand I could see that a bad ICV (or short in the harness) can damage a good DME. So there is always risk with swapping DMEs either way without doing some basic testing first.

Ingo
__________________
'74 Targa 3.6 (not stock ) - '01 C4 (almost stock) - '00 ML430 (stock)

I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
Porsche "Hammer" or Porsche PST2 - I can help!!
How about a NoBadDays DualChip for 964 or '95 993
Old 09-20-2008, 11:23 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #17 (permalink)
Author of "101 Projects"
 
Wayne 962's Avatar
Ingo, in your experience, how often is it a problem with the DME? I think that about 9 out of ten times, the problem lies in components or the harnesses, not the actual DME unit?

-Wayne
__________________
Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
101 Projects for Your BMW 3-Series 101 Projects for Your Porsche 911 How to Rebuild & Modify Porsche 911 Engines 101 Projects for Your Porsche Boxster & Cayman 101 Projects for Your Porsche 996 / 997 SPEED READ: Porsche 911 (October 2018)

Check out our new site: Dempsey Motorsports
Old 09-21-2008, 12:16 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #18 (permalink)
Registered User
 
ischmitz's Avatar
 
Join Date: May 2002
Location: Santa Barbara, CA
Posts: 4,509
Garage
Send a message via Skype™ to ischmitz
Wayne,

I agree that most times it is something else other than the DME. The DME is amazingly bullet-proof and it takes quite some abuse to damage it. Most times in loose order of priority I see issues with:

1. The DME relais
2. The flywheel sensors or CHT sensor
3. The alarm unit (if fitted)
4. Stuck injectors/fuel filter, fuel pressure, fuel pump
5. The AFM
6. The ignition switch

I think one day I have to get off my couch and write up a little troubleshooting article about no-start conditions on the 3.2 - something like a step-by-step diagnostics with simple tests.

That reminds me, when is the Pelican Wiki coming online?

Ingo
__________________
'74 Targa 3.6 (not stock ) - '01 C4 (almost stock) - '00 ML430 (stock)

I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
Porsche "Hammer" or Porsche PST2 - I can help!!
How about a NoBadDays DualChip for 964 or '95 993
Old 09-21-2008, 12:27 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #19 (permalink)
Registered User
 
burgermeister's Avatar
 
Join Date: May 2007
Location: Springfield
Posts: 1,974
Garage
If the CHT gets loused up, isn't the limp home fix to ground it (as opposed to leaving it open)? I thought grounding the CHT wire should make it run OK if the sensor is bad. My memory sucks, so don't take my word for it ...
__________________
'88 Coupe Lagoon Green
"D'ouh!" "Marge - it takes two to lie. One to lie, and one to listen"
"We must not allow a Mineshaft Gap!"
Old 09-21-2008, 06:02 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #20 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 03:55 AM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.