79 911SC Problem
 

Ok Guys,

Need some help/advice from the Pelican brain trust so here goes. My car has developed a problem with # 4 cylinder (passenger side rear) it does not seem to be firing at all.

The motor is a 3.0 CIS rebuilt with less than 2500 miles on the build.
The car has been sitting for a few months in a temp controlled garage and started from time to time. When I drove the car for a short spin it seemed to be down on power that is what got me to looking. It starts fine, idles fine and runs ok with an occasional popping noise like a backfire.

After troubleshooting it seems # 4 is not running.

Here is what I have done this weekend to try and get # 4 back in the game.

Drained old fuel from tank, replaced with new fuel.
Changed all six injectors with new Bosch ones from Pelican
Changed the spark plugs.
Tested the spark from the # 4 wire and plug, strong spark and the wire works with other cylinders.
Observed the spray pattern from # 4 injector in a jar and compared it to # 1 they are the same.
Took a compression reading and compared it to # 2 and # 3 cylinders they are the same.
Took a digital temp reading at appx 2 inches from the exhaust on the header tube it is 150- 190 degrees, the plug is black all other cylinders the temp measures 380-405 at the same location on the individual header tubes and their plugs are a nice brown/tan color.

When you completely remove the spark plug wire from # 4 there is no change on the engine at all, if you remove any other spark plug wires you notice a change in the engine.

I am at a loss here, it seems to have spark and fuel and good compression why is it not running ?

Any advice or help would be greatly appreciated.

Todd
South Georgia

No spark.

You need compression, fuel/air, and ignition. You confirmed first two, and absence of third. Black plug signals unburned hydrocarbon.

Check:
1) attachment of plug wire to plug connector
2) attachment of plug wire to disty cap
3) disty cap
4) plug gap [just in case somehow it's wildly out of spec compared to 1-3, 5-6]
5) arcing/cross-firing of #4 plug wire

Good luck, HTH

Sorry Todd, missed this. Sounds to me like you might either have mis-gapped #4 plug or (less likely but does happen every once in a great while) have a bad plug. Which plugs did you install?

Hi

I installed Bosch Supers the same ones I took out I even traded plugs between cylinders and no change on number 4 I have racked my brain today trying to figure it out.

I have a theory but don't know here goes ' "what if I had a intake valve that was not opening on number 4" I would have spark and fuel from injector and good compression but the cylinder would not fire as there would be no fuel getting into the combustion chamber hence the plug would be black and exhaust cold ???

What do you think am I off base on that, where would all the gas be going if the valve was not opening ?

Todd

Oh and gap set at .28 on new plugs

Might be stuck valve, but black plug is typically sign of under-combustion. Usually overly rich, but also can be mis-fire. Maybe switch plugs from another cylinder and see if the symptom moves? Perplexing . . .

Hi Flat

Switched plugs between cylinders to verify plug working, number 4 stays the same with new plug and the plug removed fires where ever I place it. Number 4 is completely dead does not fire at all the only thing I can think is intake valve not opening at all.

Todd

Check and double check you have the wires running from the distributor cap to the correct cylinders. (firing order 1-6-2-4-3-5)
Easy to swap 2, and the engine will run, ask me how I know... :)

[QUOTE=Flat Six;7088750]Might be stuck valve[QUOTE]

Drop the oil and remove the cam box covers.

Turn the engine over by hand from the crank and observe the valve operation.....

It is possible that the valve(s) are stuck or even not closing/seating properly. Check the clearances, of course whilst you are there......

Somethings up with # 4 Intake valve not closing I reckon.

I am ,of course, the King of wild guesses....:D

Make sure your firing order is counterclockwise on the cap.
Bruce

my first thought was going to be compression. but i see you have checked that. you might check again and also do a leak down test.

you could check the fuel volume from the injector. place all 6 injectors in containers and lift the sensor plate for about 30 seconds to a minute and compare. look for a big difference.
(could be fuel dist problem)

something blocking the intake to that cylinder.

valve adjustment. too loose.

take a look at the rest of the CIS. you could have bad air leaks on the other 5 making you compensate and run #4 too rich. look at intake bolts/gaskets, the rubber boots on the runners, (i just removed my CIS and 3 runners just fell off). injector sleeves and Orings.

anything else that can make the other 5 run lean, compared to #4.

Compression test equal across all cylinders, if the intake valve is not opening where is the gas going that the injector is spraying ?

Todd

it may not be opening enough. it was down on my list, in my head, but might be worth looking at.

Hey guy's

Ok took off the valve covers and rotated engine while inspecting for issues/problems none were noted and all valves adjustment seemed fine.

Anybody got anything that I may have missed, all the rubber boots/connectors on the intake tubes look to be "like new" and are tight with no air leaks.

Number 4 still not firing.

Todd
So Georgia

both in and out checked out? dont do like i have a habit of doing. when i go to #4 under the car, i tend to go to #5, thinking i am on #4.

fuel volume. you could have a fuel dist problem.

dont rule out having a smoke test done.

like someone above said, fuel, air, and spark. i guess you could always add compression.
i think i would look at doing a leak down test too.

blocked intake runner.

go back over what you have done. you may be doing something wrong. also, all too often we are not told everything that has been done or the history until it is fixed.

I am taking a break from it for a couple days I have a track weekend in Savannah after that I will get back into it, I am going to pull number 4 injector and lift the throttle plate this time when I checked spray it was at idle.

I will need to pull the runner on 4 to make sure it is open but I don't see how it could have become blocked but I am willing to try anything at this point.

Todd

Are the spark plug hole threads (in the head) clean for #4? Too much anti-seize or other stuff (even oil/carbon etc) in the threads will keep the plug from firing. Some anti-seize is good - too much is TOO MUCH. It will keep it from grounding.


angela

Have you pulled the distributor cap and inspect and look for any carbon tracking between posts?

Hello everyone,

Ok, finally figured it out. Would like to thank all who replied that is what makes this board great.

So the culprit is the fuel distributor, (or seems to be). I performed a leakdown on # 4 everything was fine on that. I swiched the injectors with line attached from # 4 and # 5 and low and behold # 4 fired right up and # 5 went cold. For whatever reason the FD is not supplying enough fuel or enough fuel pressure to # 4 cylinder.

So it is off with the FD in the morning and to the bench, will keep everyone posted.

Todd

Take the line off and make sure it is clear before pulling the distributor....
New lines are available from our host or other sources like gowesty supplies both types of hose.(For both sides)
Maybe the distributor is clogged up. Some good threads on rebuilding them are posted.