linderpat

Have a longstanding issue that I am finally getting addressed, but even the experts are stumped. My car, Blumaxx, a 78 US model, stumbles at WOT in the higher rev range. If you do not push it that hard, you would never know it. It runs great, except when under full and near full load. I have ruled out many things that could be the culprit - new plugs, wires, rotor, coil, green wire. Clean grounds. I have installed a few years back the short MB injectors, but it did this before I replaced the original injectors (I made the swap initially because of the stumbling issue). The original injectors have been R&R'd, so they will likely go back in.

I took the car to my local indy shop. The guy there is a wizard - serious Porsche specialist (factory certified) and knows 928s very well. He is a CIS expert.

He initially found issues with the fuel pump relay, and the fuel pump fuse connections at the CE board, and was hopeful that was the problem. He remedied those things, had some improvement, but the stumble is still there. He ruled out ignition issues, and says it is fuel starvation. He is now testing pressures, etc.

Anybody have any thoughts? This is for the K-jet/CIS experts. Way beyond my pay grade at this point

SwedeInSiam

Is the vacuum hose to the WUR connected correctly and work properly (if the '78 have it same as RoW S from '80). Will give extra fuel at WOT.

GlenL

Often a stumble is electrics but I'm going to suggest fuel supply. Electronics aren't affected by WOT versus partial throttle.

Something you may benefit from is a dyno test with F/A sniffer. This would show if you were running lean and going too lean. That's just a hunch. It would also show that the mixture is good but it's stumbling and that'd point back to electronics.

So...

Is the fuel pump correct and is it working right? The CIS needs a higher pressure than other P-cars and swapping in the wrong but available fuel pump could be a problem.

When was the last fuel filter change? Sounds obvious but worth a try.

Have you cleaned the screen at the inlet to the fuel distributor? It looks like a banjo fitting adapter but there's a cone-shaped screen inside. The last defense before flakes and gunk gets in the precision works for the FD.

You could try a flow-rate test. That's messy and doesn't really prove much, IMHO. The fuel pumps generally run correctly or not at all.

Have you run much good fuel system cleaner through it? I like Seafoam and Techron.

Chuck Schreiber

Had the same exact issue w the SilverBulit.
Sean went through the entire fuel system, fixed in 2 days.
Now I can get in it after 4 weeks of sitting, fires right up, no stumble, no hesitation..
Think he rebuilt the fuel dist, pulled/cleaned all screens, adjusted control/system pressure, etc...
I've owned the car since 95 and it's NEVER run so good..

linderpat

Yes, new fuel filter (last one was replaced about 6/7 years ago, and it was clean when I cut it open)and we just added two new (and correct) fuel pumps. CIS system was gone through two seasons ago, and cleaned, although generally my car is very clean to start with. Use Techron regularly and high quality e-free fuel for the most part. My tech did clean out the sediment from the tank, and he said there was a fair amount in there (he drained and flushed the tank).
Also, the car always fires right up, idles steady and strong. If you didn't get on it in the higher range, you would not know there was an issue. This is a real puzzler.

SeanR

I agree with the post above about getting to a dyno to find out what the AFR is doing, it's going to be a quick and cheap way to do some diagnostics.

kary4th

I had the same symptoms once, changed the fuel filter & performance improved but issue wasn't resolved.

Removed the (broken) in tank fuel filter and then replaced the inline fuel filter a second time.

The problem was resolved.

linderpat

all good stuff; keep it coming.

Jadz928

If your CIS expert is about to test flow & pressure, then he is just getting started.
Report back with flow-rate & pressures. Otherwise you're just spitting in the wind.

9two8

Had the exact same symptoms a while back, the stumble at higher revs and the car seemed to jump while this happened, replaced wires, cap, plugs, checked vacuum lines, basically my man that does for me did everything imaginable. The last thing was to strip, clean and rebuild the fuel distributor, but he found a big vacuum leak where the F/D attaches to the engine, found with a squirt of appropriate fluid round the base, ( Touch wood ) all well now and running really strong, especially after replacing every vacuum line on the car, even the big half inch one from M/C to inlet. Incidentally don't know what importance but the M/c vacuum line to the spider has a black tapered plastic jet inside the pipe that has a small hole through it ( my pipe cleaner fitted perfectly ) this hole was blocked solid, so guessing there would be vacuum restriction there. Just throwing this info out to you Ed with some hopeful pointers for you ?

linderpat

My mechanic finished the evaluation and the tweaked the k-jet pressures. The car has never run like this before - like a scalded dog! Damn it is responsive. It was stumbling under load and high RPMs. Normal driving did not show any issue, but I drive hard, so there it was. Car was running lean - fuel starvation at load. The issue ultimately was with the system fuel pressure - it was set 25% below factory spec. My guy likes it about 10% above. He made the appropriate adjustments, and also fixed my longstanding and lingering idle issue. His words on invoice - "Runs perfect." I can tell you it does

Other things fixed - burned fuel pump fuse contact repaired, and new fuse and relay (old relay was getting pretty toasted). New fuel pumps (X2), drain and clean fuel tank. I should be good to go for awhile now.

linderpat

Fuel pressure was 1.5 bar low according to the work order. R&R fuel pressure valve with spring and shim, and set at 6.2 bar. Also adjusted the decelaration valve to correct idle issue.

Chuck Schreiber

Glad to hear!

When CIS is in order, life is good!!

The Deputy

Late to the party, but I'd have headed for the fuel delivery from the get-go. Usually, a stumble at WOT is fuel related. Granted, there are plenty of potential problem areas with this system.

Glad she's running strong and true.

Brian.

crushingday

Is there a write-up somewhere that can help me check this myself on my US '78? or do I really need to take it to a CIS expert?