Engine Missfire
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Joined: Jul 2005
Posts: 83
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From: Grimsby UK
Engine Missfire
About a month ago my engine started to missfire under heavy acceleration. The sound is very deep like a dog barking (big dog!) and appears to be at a frequency where one or maybe two cylinders are affected. I have had the engine checked on the Porsche diagnostic computer and no faults were reported. There are no warning lights showing. I have checked the plugs, leads, distributor caps and rotor arms. I have cleaned the HT coil tops and I have checked the exhaust and heat exchangers for leaks. When the heat exchangers were removed I looked into the exhaust ports and the exhaust valves looked ok, but a bit sooty. The petrol consumption is not as good as previously as it averages 22mpg around town normally and has dropped to 19mpg.
Could it be the injectors? The car has done 158 000 miles on the originals. Are they easy to remove for cleaning and do I need to get new seals etc?
Has anyone any other ideas as the problem is getting worse and now happens under normal acceleration.
Could it be the injectors? The car has done 158 000 miles on the originals. Are they easy to remove for cleaning and do I need to get new seals etc?
Has anyone any other ideas as the problem is getting worse and now happens under normal acceleration.
Drifting
Joined: Jul 2005
Posts: 3,363
Likes: 25
From: NY, USA
Are the coils original?
It appears that these are beginning to fail more frequently. I've replaced mine due to poor running and now my mileage is up from 19mpg (US) highway to 24mpg and it runs stronger and smoother.
Have you tried starting with each coil independently?
It appears that these are beginning to fail more frequently. I've replaced mine due to poor running and now my mileage is up from 19mpg (US) highway to 24mpg and it runs stronger and smoother.
Have you tried starting with each coil independently?
Drifting
Joined: Jul 2005
Posts: 3,363
Likes: 25
From: NY, USA
Unplug them, one at a time, at the switch, which is mounted on the fronts of the coils. Then try to start the car. If it fails to start, or runs rough it would be time to replace them.
They are ~$50 each on pelican. The OE versions are ~$75, but it appears that all are now coming from Brazil which is said to be inferior technology. My new ones are silver. The OEs are black. I'd try and find the OEs, though it may be difficult to do.
They are ~$50 each on pelican. The OE versions are ~$75, but it appears that all are now coming from Brazil which is said to be inferior technology. My new ones are silver. The OEs are black. I'd try and find the OEs, though it may be difficult to do.
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Joined: May 2007
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From: Society Hill, SC
You may have already tried this and I know it's a reach but, just before my DME rely finally gave up the ghost last month, I was having back fires on start-up along with an occasional slow or no start.
As it died it's slow death, it started backfiring on the track, then just stopped. A new rely, and all's well.
As it died it's slow death, it started backfiring on the track, then just stopped. A new rely, and all's well.
Nordschleife Master
Joined: Sep 2004
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From: not in HRM
"Unplug them, one at a time, at the switch, which is mounted on the fronts of the coils. Then try to start the car."
Or while the motor is running , reach in and disconnect the plug from the switch/ignition unit , one at a time and see what the motor does .
Check the compression and or leak down for a leaky exhaust valve .
Check the exhaust valve springs for a broken spring .
Check for a crossed mispluged plug wire .
How old are you plugs ?
How old is the O2 sensor ?
"Are they easy to remove for cleaning and do I need to get new seals etc?"
How long is a piece of string ?
Its a bit of a job to pull the injectors , but not so bad .
If you get the injectors cleaned tested / rebuilt , they should come back to you with new seals .
Or while the motor is running , reach in and disconnect the plug from the switch/ignition unit , one at a time and see what the motor does .
Check the compression and or leak down for a leaky exhaust valve .
Check the exhaust valve springs for a broken spring .
Check for a crossed mispluged plug wire .
How old are you plugs ?
How old is the O2 sensor ?
"Are they easy to remove for cleaning and do I need to get new seals etc?"
How long is a piece of string ?
Its a bit of a job to pull the injectors , but not so bad .
If you get the injectors cleaned tested / rebuilt , they should come back to you with new seals .
Thread Starter
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Joined: Jul 2005
Posts: 83
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From: Grimsby UK
Compression check is ok. Plug wires are all correct. Plugs 6 months old. O2 sensor 5 years old but checks out ok as per the readings from the Adrian Streather book. I will have to wait until weekend to do the coil checks as there is not much daylight at present.
Thanks for the suggestions.
Thanks for the suggestions.
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Drifting
Joined: Jul 2005
Posts: 3,363
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From: NY, USA
Here's a thread on injector cleaning.
If you go this route I'm told you can run the fuel rails dry while the car is running by pulling fuse #34 for the fule pump. It saves spilling fuel when the injectors are separated from the rails, though, it was not a problem to catch the spilled fuel with a small towel.
Someone please chime in if it's not a good idea to pull the fuel pump fuse!
If you go this route I'm told you can run the fuel rails dry while the car is running by pulling fuse #34 for the fule pump. It saves spilling fuel when the injectors are separated from the rails, though, it was not a problem to catch the spilled fuel with a small towel.
Someone please chime in if it's not a good idea to pull the fuel pump fuse!
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Burning Brakes
Joined: Feb 2003
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the misfire may be a bit subjective but I had what may have bee interpreted as a misfire which appeared at first only on heavy acceleration but over time gradually appeared on moderate acceleration. Compression tests showed a slightly low compression on #6 but when I did a leak down test it was obvious #6 was leaking with a 10% reading. I retorqued the head bolts - the leakdown came down to 1% and the misfire dissapeared along with the slight oil weap I had on #6 cylinder base. That was 6 months ago and all is still good.
Just a thought
Bill
90C2
Just a thought
Bill
90C2
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Joined: Aug 2007
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From: Cape Cod, Ma
You can check the coil resistance with a multimeter. Primary circuit black lead to negative terminal, red lead to positive.Should read 0.4-0.6 ohms. secondary circuit Red lead to coil center, black lead to negative terminal.
Should read 5-7.2 ohms.
Should read 5-7.2 ohms.
Addict
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From: Atlanta
Mine ohmmed out fine, but I was hearing a spark in the radio and could not find the source. Car ran very poorly. Radio and car were both fine when the coils were replaced.
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Joined: Jul 2005
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From: Grimsby UK
I took out the DME relay and fitted jumper leads and everything ran as before so I assume that the relay is ok. I carried out the coil static check with a multimeter and everything is ok. I tuned the radio to a quiet station and revved the engine and there was no interference. I disconnected one coil and the engine ran ok but the missfire was not as bad. I disconnected the other coil with identical results. Connecting both coils brought the missfire back at its worst.
Am I back to pulling the injectors?
Am I back to pulling the injectors?
Nordschleife Master
Joined: Sep 2004
Posts: 5,061
Likes: 2
From: not in HRM
I would stay with the spark . When you say the "misfire was not as bad" , that makes me think its a misfire from a lack of spark .
Have you ever done a spark color test before ?
Why would one spark fire off a cylinder when two sparks don't ?
Is it not able to make the two sparks at one time ?
Yet somehow able to make one good spark at a time ?
Or am I just running up another blind alley ?
Have you ever done a spark color test before ?
Why would one spark fire off a cylinder when two sparks don't ?
Is it not able to make the two sparks at one time ?
Yet somehow able to make one good spark at a time ?
Or am I just running up another blind alley ?
Thread Starter
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Joined: Jul 2005
Posts: 83
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From: Grimsby UK
Thanks for your comments Indycam.
As far as the spark goes the only thing I could think of was that the HT leads were shorting out somewhere. They are only 18 months old and I have checked them today and they look ok. I also ran the engine in the dark and looked for HT sparks but could see nothing.
The difference in the misfire on one coil is very obviously better. With two coils operating I can get a misfire at any revs simply by prodding the accelerator. With either coil disconnected I need to take the revs to about 2000 rpm and give the accelerator pedal a real firm shove to get a misfire about half as loud as with two coils.
I am running out of ideas!!!
As far as the spark goes the only thing I could think of was that the HT leads were shorting out somewhere. They are only 18 months old and I have checked them today and they look ok. I also ran the engine in the dark and looked for HT sparks but could see nothing.
The difference in the misfire on one coil is very obviously better. With two coils operating I can get a misfire at any revs simply by prodding the accelerator. With either coil disconnected I need to take the revs to about 2000 rpm and give the accelerator pedal a real firm shove to get a misfire about half as loud as with two coils.
I am running out of ideas!!!
