Misfires and low compression
07-18-2017, 06:25 PM
#1
2nd Gear
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Misfires and low compression
Hi all,
i recently purchased a 95 911 on 38k miles and after doing some background digging I found the following diagnosis report that I am sharing here. I haven't had the engine light come on yet whilst I have owned the vehicle but admittedly I want to get to the bottom of the issue and resolve the problem as the car is very nice and I'm hesitant to drive it without resolving this. Car has sat in storage for 3 years with little attention or use.
theres plenty of misfire information on this forum which I have been scouring but what worried me about the diagnosis that was performed was the loss of compression on one of the banks.
im after some advice on which order to approach the usual suspects before I start buying replacement ignition components and working through it that way. I'm most concerned about valve train/top end issues. Vacuum ignition fuel etc can be solved fairly easily within the scheme of things following instructions on this great forum.
greatly appreciate any advice on the following diagnosis from a non-Porsche specialist
---------------------------------
Test drive and can not verify that the SES light comes on Hook-up external power supply
Hook-up PWIS
Scan for fault codes in the DME
Found that all 6 cylinders have mis-fire counts
Also you can hear the engine is mis-firing
Hook-up lab scope to the ignition system to check burn time and look for ignition mis-fires Found that ignition components to show signs of deterioation
Hook-up smoke machine to check for vacuum leaks
Found no leaks
Test engine vacuum found that there seems to be low engine vacuum 12 lbs
Possible issue distributor ignition timing off from main distributor to the 2nd distributor Gain access and inspect - ok
Perform compression test found that the right bank is about 25 psi lower than the left side Most likely there is a timing issue - no further testing was done
i recently purchased a 95 911 on 38k miles and after doing some background digging I found the following diagnosis report that I am sharing here. I haven't had the engine light come on yet whilst I have owned the vehicle but admittedly I want to get to the bottom of the issue and resolve the problem as the car is very nice and I'm hesitant to drive it without resolving this. Car has sat in storage for 3 years with little attention or use.
theres plenty of misfire information on this forum which I have been scouring but what worried me about the diagnosis that was performed was the loss of compression on one of the banks.
im after some advice on which order to approach the usual suspects before I start buying replacement ignition components and working through it that way. I'm most concerned about valve train/top end issues. Vacuum ignition fuel etc can be solved fairly easily within the scheme of things following instructions on this great forum.
greatly appreciate any advice on the following diagnosis from a non-Porsche specialist
---------------------------------
Test drive and can not verify that the SES light comes on Hook-up external power supply
Hook-up PWIS
Scan for fault codes in the DME
Found that all 6 cylinders have mis-fire counts
Also you can hear the engine is mis-firing
Hook-up lab scope to the ignition system to check burn time and look for ignition mis-fires Found that ignition components to show signs of deterioation
Hook-up smoke machine to check for vacuum leaks
Found no leaks
Test engine vacuum found that there seems to be low engine vacuum 12 lbs
Possible issue distributor ignition timing off from main distributor to the 2nd distributor Gain access and inspect - ok
Perform compression test found that the right bank is about 25 psi lower than the left side Most likely there is a timing issue - no further testing was done
07-19-2017, 02:30 PM
#2
RL Technical Advisor
Hi,
Without a detailed history of the car, I would perform a leakdown test followed by the 30K service in order to get an accurate baseline.
I would replace:
Plugs
Caps & rotors
Plug wires
Fuel & air filters
Clean the ISV
Check O2 sensor to make sure its functioning correctly.
You may need to run some high quality fuel injector cleaner through the system; I like the cocktail of LubriMoly Jectron and Ventil Sauber, together.
Hope this helps,
Without a detailed history of the car, I would perform a leakdown test followed by the 30K service in order to get an accurate baseline.
I would replace:
Plugs
Caps & rotors
Plug wires
Fuel & air filters
Clean the ISV
Check O2 sensor to make sure its functioning correctly.
You may need to run some high quality fuel injector cleaner through the system; I like the cocktail of LubriMoly Jectron and Ventil Sauber, together.
Hope this helps,
07-19-2017, 03:00 PM
#3
RL Community Team
Rennlist Member
Rennlist Member
In the interest in not throwing cash at a bunch of issues, I would start with the simple inexpensive stuff first.
Check distributor belt then
1, caps and rotors
2, clean idle valve
3, Air filter
4, fuel injector cleaner
5 Check O2 sensor
6. Verify no vacuum leaks
See if the above solves your problem then if not,
Pull a plug and look at it.
If looking bad replace plugs as needed.
Check wires replace if needed.
Fuel filter last as least likely.
My Carrera has only thrown a misfire code once in its 100,000 mile history. It was a loose distributor cap, go figure. I have also never replaced the fuel filter, lucky? Smart? I take lucky over smart any day...
Check distributor belt then
1, caps and rotors
2, clean idle valve
3, Air filter
4, fuel injector cleaner
5 Check O2 sensor
6. Verify no vacuum leaks
See if the above solves your problem then if not,
Pull a plug and look at it.
If looking bad replace plugs as needed.
Check wires replace if needed.
Fuel filter last as least likely.
My Carrera has only thrown a misfire code once in its 100,000 mile history. It was a loose distributor cap, go figure. I have also never replaced the fuel filter, lucky? Smart? I take lucky over smart any day...