hesitation/slight missing while cruising
 

Hello,

I have a problem while cruising at a steady speed most noticeable around 2500-3500 rpm. Its like a couple second hesitation or slight miss, I don't think I would describe it as a surge (but I could be wrong). Engine pulls strong all the way through if I am heavy on the throttle and doesn't seem to do it over 4000 rpms if just cruising. The tach doesn't move, it will remain at 3000 or whatever rpm I am cruising at but you can feel the missing/hesitation as a vibration.

I have been trying to fix this problem by adjusting the mixture by ear without any luck. It idles fine at around 950 rpm.

I just re-checked my fuel pressures and they seem off from the info I have found.

System pressure with valve closed and wur unplugged 6 Bar

Valve open with wur still unplugged pressure drops to 1.3 Bar

Plug wur in and pressure slowly rises to 3.8 Bar

Engine warm with wur plugged in and valve open 6 Bar

Any suggetions appreciated

The Car is 1978 ROW 911SC with 964 cams

Thanks
Craig

When was the last time you changed the fuel filter?
Your pressures don't seem to be too far off.
How cold is it when you are measuring the WUR cold unplugged pressure. (Seems a little low)
Have you lubed the distributor lately? That is about the rpm range for the advance mechanism to start kicking in.

Fuel filter is new. I have never lubed distributor, any tips?

I was thinking system pressure should be between 4.5-5.2 and I am at 6 bar?

To lube dizzy PPP off rotor and drip some 30 weight oil on the felt. Get it saturated.
Not sure on the fuel pressure being high, restriction in the return line?

Ok, I'll give that a try.
Thanks

I had a very similar thing and it turns out to be an intake gasket leak. when I get the car back I will let you know if it is fixed.

I have been going through a similar thing with my '78. but it was more than a hesitation. I would loose power then a huge backfire! Today, I just change the CDI over to MSD and so far have seen a great improvement. Not sure if this is similar to what you are experiencing but that could be the beginning of the CDI failure.

Just a thought.

Ok I lubed the distributor. I will also double check the intake runners as I did have the cis off recently.

My 78 SC is at a local shop for this exact same issue. Grenades the distributor last July but that was replaced. This mysterious hesitation/stumble/miss or whatever you would like to call it has the techs stumped. CDI box swapped, wires, harness, etc has been checked with no luck.

I will mention in intake gasket possibility. Very interested to hear what everyone's own experience will be regarding the fix for their cars.

Cheers

When did this issue arrive? Any idea why? What other changes have you made, if any?

I like your thinking -- I'd start by fixing the fuel pressures and move on from there. I would dial the system pressure to 75psi by playing with the shims on the regulator. Do you have a copy of the Probst book? Too much fuel pressure is just as wrong as too little.

I'd be checking AFR's (CO%) at cruise RPM to see if you have a fuel or ignition problem.

What plugs are you using? How old are your plug wires, cap & rotor?

I just reinstalled engine after having top end apart to replace lower head studs.
Cap and rotor are about a year old, the plugs are new Bosch (don't have the number in front of me), wires were replaced about a year and a half ago

I also replaced all the injector o-rings and sleeves and intake runner rubber and gaskets

Do you have any additional updates of your issued?

3.8 is on the hi end and 6bar is hi. these both will lean the mixture.

the 2500-3k miss on the CIS 3.0 seems to be a common problem, especially when something is not right.
air leaks is first thing. intake bolts, vacuum lines are some things to check.

you also have the vacuum enrichment WUR. check the control pressure with vacuum (about 3.6bar) and without vac( about .8 bar less).

then its a matter of mixture and timing.
try moving the timing up and back 5 degrees and see if it gets any better.

richen the mixture as much as the idle will allow.
IE. keep adjusting it richer until at least one of these 2 things happen.
it surges at idle
or when you rev it up and let the idle come down, it dips down below idle then comes back up.
you want it right on the edge of this happening.

Problem has been solved!!! It turned out to be a partially clogged injector on cylinder 4. It was spraying somewhat sideways and seemed to require some addition pressure than the other injectors to spray so I am assuming that cylinder 4 was missing when heavy throttle was not being used.

I tried cleaning it with some carb cleaner and compressed air but it didn't work. I replaced that injector with a new one and the hesitation/miss is gone.

I agree the fuel pressures are still higher than usual but the car seems to be running good now.

Thanks for all the suggestions. I am slowly getting to know my engine pretty well :)

Congrats on locating your fix. Will need to check out the same on ours. Will need to check for possible air leaks and/or blocked injectors.

Cheers

Her is one for the "strange and unusual" regarding these symptoms.....

Our 78 911SC we're exhibiting the exact same symptoms. The cause of this great mystery......

The EGR valve. Who would of thunk. Just one more thing to check when trouble shooting this type of issue.

Cheers Trevor


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