moalaska

Im having issue with what seems an rpm specific fuel cut

At about 3k rpms afr goes high to 16 and starts bogging bucking, 1/3 throttle or worse

Symptoms seem similar to bad tps but dont think its tps

Afrs spot on idle and no load cruising
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Fpr set to 45 psi, but when warmed up pressure dropped has to readjust

Tried unplugging o2 sensor, same symptoms
Unplug tps, same symptoms just worse
Raise fuel pressure a little, symptoms still there
Adjust maf piping
Run dedicated power and ground to maf
Set idle per procedure

New fuel filter, check valve, spark plugs, newer cap rotor

Replaced ref and position sensors
Running msd blaster coil with ballast resistor
Resolsered dme within last year, cleaned all grounds as well


Could it be fuel pump going out
Maybe tps is flaky
Bad dme or injector drivers?

Thoughts

Humboldtgrin

I would start on the fuel pressure first. When was the fuel filter replaced? Have you checked the hard lines to make sure there not pinched? Is the timing belt installed correctly?

moalaska

Fuel filter replaced within last couple hundred miles along with fuel check valve and cleaned tank screen. Haven't checked hard lines. Timing belt done within last couple hundred miles as well with rollers and balance belt and timing marks line up.

NCLA951

Having similar issue, mine is worse when cold. Have to nurse it along if I give it too much throttle it will bog but if I baby it I can get around. Entirely new fuel system and flow checked. When I get back from Xmas holiday I'm going to install the new ref/speed sensors and harness and see what happens.

Please post if you figure yours out!

Humboldtgrin

you need to check the fuel pressure when that is happening.

moalaska

Kind of hard to check Fuel pressure while driving but i agree that would be nice to know.

MAGK944

Have you checked you vacuum lines?

moalaska

Ive redone all the vacuum lines ill double check though.

rlm328

Check your IC piping connections, sometimes the silicon hose that connects them will pop off. You may want to use T-Bolt clamps downstream of the turbo.

moalaska

Ok so just installed the mercedes fuel pump in the 944. So far my fuel system is all updated, pump, filter, check valve replaced tank screen cleaned. Aftermarket fpr with check valve installed after fuel rail and before reg. 54# injectors from vts. Still having issues. At about 2800 rpms, at light throttle cruising, afrs go to 16-17, with stumbling. Pulled the plugs to look at them. They have about 50 miles on them. Not really sure where to check from here. I haven't changed my spark wires yet, maybe at that specific rpm range they arent working good. Rotor and cap are New. Im about to go crazy and order a vems system then i can see what all my sensor outputs are doing.

moalaska

Ok decided to check timing. Set crank at tdc looked through bellhousing hole ot in middle of hole. What's wrong with this??

moalaska

Nevermind. Rotated it again and its spot on.
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Tom M'Guinn

Are you sure the motor is at TDC #1 on the compression/combustion stroke? From the position of the rotor, you may need to rotate the motor one full revolution to get it where you want it, to confirm the cam mark lines up.

Edit: Posts crossed, you got it.

Tom M'Guinn

Bad wires are a notorious source of stumbling under load. If you aren't confident in yours, I'd change them. If you feather the throttle, can you get the RPMs up above 3k, or does it cut out at 3k no matter how soft you are on the throttle? If it cuts out at 3k no matter what, then you might double check the TPS idle switch at the DME, including the wiring and DME, to see if the DME is seeing a closed throttle and refusing to deliver fuel. If you can feather it gently up the RPM range, then weak ignition is a possibility.

Looks like a lot of rigmarole on the fuel lines/damper/fpr. What are you trying to accomplish with all that, and are you sure you are not creating issues you could avoid with a good old Bosch 3 Bar FPR on the rail?

Do you have the MoMonitor for your Vitesse system? It let's you monitor and log most of the engine sensors/vitals while driving.

Have you checked the solder joints in the KLR? Have access to a spare DME and KLR you can try?

moalaska

Another question
I did google search and found results for 911s but not 944. I decided to check my rotor for continuity since it was off. Found that it doesn't have clean continuity there is resistance, so either rotor has built in resistor or its bad. I know spark plug boots have built in resistance too. Do i have wrong rotor seems weird that there would be resistance in rotor and boots, but maybe they put resistor in rotor to reduce chance of arcing inside the cap or something.