Ok...I give. HELP! (With fuel/ignition issue)

[rennch]

Hi all. Myself and a friend have been trying to figure out this issue the entire day. I'm $300 leaner now, and no closer to a solution. Here is the issue.

THE SPECS:
72 911, 79 3.0 SC motor with stock CIS.

THE SYMPTOM(S):
Engine acts like it is either misfiring, or losing fuel pressure under load, above around 3000 RPMs. Getting random tufts of smoke from the exhaust, for a couple seconds at a time. Symptoms are seemingly worse when engine is hot. It kind of came out of nowhere...the car felt like it was starving from low fuel one day, and it has been acting like this ever since. The only anomoly with the car is that the inner bracket that holds the fuel filler nozzle fell *into* the fuel tank a couple of weeks ago. The engine seemed to run ok after that though, until now.

WHAT WE'VE DONE:
Initially, we thought it was an ignition issue, so here's what we've changed.
1. New spark plug wires
2. Checked cap and rotor
3. Replaced Permatune and stock coil with MSD 6AL and Blaster 2 Coil
4. Checked and regapped spark plugs for MSD
5. Replaced fuel filter

This was all done today...and nothing changed a thing. So....

CURRENT SPECULATION:
1. Perhaps some sort of fuel breather issue? I don't know much about the fuel venting system, but I'm wondering if there was some sort of nut from the bracket that formerly held the fuel filler bracket that fell into the fuel ventilation system and is clogging something.
2. Something up with the fuel pump? Any way to test it for a definitive answer?
3. I think we covered all the basis with the ignition, so any other suggestions would be appreciated.

Anything I've missed?

Thanks for any information you might be able to provide.

Thanks!

Michael O'Neal

[Mookster]

In the SC's with CIS systems there is a relay under the passenger seat next to the CIS computer. I am not sure how the upgrade of the 3.0 motor is wired in your vehicle.

If the engine is hot and the vehicle is hicupping (bucks like like a 16 year old learning to drive a 5 speed) replace that relay ($32). I would also suggest replacing the fuel pump relay ($22) and the vacuum hoses hoses ($15-20) as well.

Side items to consider:
1. flush the fuel tank
2. Clean the electrical connections of the fuel pump relay
3. Replace the fuel accumulater
4. bench test the fuel pump, replace if bad
5. Alt?

As a side note my CIS computer crapped on me. Rule out the cheap things first.

[Paulporsche]

Check your control pressures, especially warm. It could be you are leaning out when warm because of your WUR.

[rennch]

Mookster,

I don't think I have that relay...this motor is in a 72, so it's likely that was bypassed at some point. Also, I'm curious how to bench test the fuel pump?

Paul...how do I "check my control pressures"? What do I need to do such a thing?

Thanks again...

[Argeo]

Does the 79 SC have points? It sounds like what mine was doing, adjusted points and problem disappearred.

David

[rennch]

It did at some point. It's got a pertronix setup now. I don't know if that setup fails very often though.

[Argeo]

Well good luck. It sounds to me like power (electr) is cutting out for some reason to generate spark. It then sparks again resulting with the backfire. Did you pull a fuel injector and give a simple test to see how much it is spraying? It shouldn't just squit out but spray with good pressure behind it. (sorry not to good on details).

What I did was go see my favorite wrench (Kevin at Randolph Racing for all you eastern MA guys) and he diagnosed my problem in like 5 mins (if that). I learn as I go and didn't even know what points were then. I hope I do not have to learn about headstuds anytime soon.

Good luck,

David

[Paulporsche]

Emptyo,

If you search you will find lots of info on this. You will need a fuel pressure gauge and you hook it up to the fuel line that runs to the WUR.
You will get a reading that you can compare w/ what your engine should have. The control pressure regulates how rich your mixture is. Before you get the gauge, you can try richening the mixture slightly, and driving the car. If the "miss" goes away, then fine, you were too lean. If it is worse, lean the mixture slightly and drive. If there is no change, put the mixture setting back. When you change the mixture, you will probably have to adjust the idle speed too.

Do you have any other symptoms?

Have you checked that your distributor shaft turns and snaps back quickly?

Have you checked to see if you have the DME relay that Mookster mentioned? Also, even though you have new wires, make sure they are tight inside the connectors and that the connectors are firmly seated in place.

Do you have a popoff valve? Is it seating correctly? Check the vacuum line hoses to see if they are on tight.

Does this symptom come on while under load, or while revving the engine?

[jpnovak]

79 3.0 does not have any relays with it. It will also not have points since there is a magnetic pickup trigger. These areas are not your problem.

I suspect a fuel problem. Check the fuel pressure as indicated above. I would also remove and clean all contacts to the fuel pump. It may be ailing when it gets warm.

Have you checked the battery voltage at speed when warm? Have you checked the trigger wire from the dist to see if its cracking/worn? both of these items would lead to ignition cutout at higher rpm, especially when warm.

[Paulporsche]

Since you have no DME, as Jamie says, then you do not have an O2 sensor. What about the condition of your Cat?

You might try a less aggressive plug gap and see what happens.

When did you last have a valve adjustment?

I would check the dist, try the mixture adjustments, and test the plug wires, and the fuel spray pattern of the injectors.

[Argeo]

Just a reminder, if you are going to adjust the fuel Distributor plate, make sure the car is warmed up. I would turn the engine off when adjusting incase of backfire or something else, though my wrench did it with engine running. Also, make sure (double make sure you remember how far you have turned that mixture screw and turn in 1/4 intervals or less, this way you can always go back to the original starting point. It has been a while but I believe clockwise richens, ccw leans. Please someone correct me if I am wrong on the direction. Don't forget about the adjuster on the throttle body. Direction is reverse to plate screw. Getting a fuel pressure gage is probably the best idea. I don't have one but I enjoy making myself crazy. Also, I am still thinking electrical but I blame everything on electricity so take my opinion with great caution. Heck, I am replacing my power windows with manual because I have a alignment problem .

Good luck again,

David

[Early_S_Man]

Michael,

Above 3000 rpm is the speed at which an MSD box switches over from multiple sparks to single spark per plug ... and failure of the switching section logic can cause similar rough running, or failure to run at all above 3500 rpm to 4000 rpm!

About the only conclusive way to troubleshoot the ignition is to either put the engine on a Sun Analyzer ... or use an inductive-pickup probe from a timing light or automotive DMM connected to a traditional oscilloscope, then rev through the rpm range where the problem shows up and look for lost or intermittent ignition sparks.

[rennch]

Warren,

We just switched to the MSD box *because* of the issue, so it's definitely not that. Definitely *leaning* (pun intended) towards a fuel issue at this point. But I'm open to any other suggestions. I don't have a fuel pressure gauge, so I'm going to try to locate one.

Thanks

[rennch]

Also...Paul, yes, this happens under load. It seems to be ok when idling, and revving whilst standing still. It stutters quite a bit when under load though. I don't know much about troubleshooting fuel systems though, so I'm trying to search on it now.

[CliffBrown]

Mike O'Neal (emptyo)

I sent you an email concerning the bad crankshaft you sold me, can you please reply.