88 M6 suddenly hesitates under hard aceleration, how to tell which fuel pump is bad?

[Lotsobmws]

My 88 M6 suddenly developed a hesitation under hard acceleration. Feels like fuel starvation. Put a spark checker on it while diving and no breaks so I'm fairly sure the ignition system is not the culprit.

Is there a way to know which pump - lift or in-line - is the problem? I'm going to rig up a mechanical fuel pressure gauge and drive it tomorrow. But if pressure drops under acceleration along with the hesitation, is there any way to tell which pump is causing the problem? Or do I have to replace both of the overpriced pumps to be sure?

Any advice appreciated.

Thanks...Steve.

[RSheiman]

First thing everyone will say is check for vacuum leaks. In tank pump can be checked with a multimeter, high pressure pump you can hear at passenger rear quarter. Latter pump usually dies as an all or none thing. I would still check cap and rotor, you checked spark but not under load and a tiny cap crack is enough to cause your sx's as it was mine.

[Lotsobmws]

I tee'd in a manual pressure gauge, zip tied it to the wiper and went for a drive. Fuel pressure was right on the money at 42 psi. It stayed there at all RPMs. This is correct, right? Under hard acceleration it started bogging again. Not like it's dropping a cylinder or two but more like it's dropping all cylinders and bucking. Seems to happen only over 3,000 RPM.

I replaced all vacuum lines when I replaced the head gasket last fall, so I don't think that's the problem. I also replaced the cap and rotor so those are new. Spark plugs are new, too

I also double checked the large gauge positive lead and that's clean and tight. The large gauge ground wire to the block is clean and tight.

Anyone got any ideas on what might be causing the car do drop all cylinders under hard acceleration? Any tests you can recommend?

Thanks...Steve.

[e24mpwr]

That happened to me, and it turned out that the harness-->ECU connector was loose.

[Lotsobmws]

Just checked my ECU harness. Was clicked in tight. Removed it to look for any oxidation/corrosion. Nice and shiny.

Here's a thought. My e36 M3 did something similar. Would buck under hard acceleration, but worse than this and only when warm. Turned out to be a bad crank position sensor. The CPS was new (1 month old and installed by a dealer for the prior owner) and tested OK - 550 ohms. However, even though it was new and tested fine, it went into failure mode when warm. Worked fine when it cooled back down. I was able to use real time telemetry through the ODB-2 to find the problem. Replaced the CPS and all has been well for 3 years.

Anyone heard of an S38B35 CPS doing the same thing?

Still scratching my head.

Steve.

- - - Updated - - -

How many crank sensors does my 88 M6 have? I see the pulse generator mounted above the A/C compressor that reads the missing tooth on the crank wheel. But does it also have the 2 sensors on the bellhousing?

I'm fighting this bogging/bucking problem and am thinking it's CPS related. Need to know how many sensors read crank orientation. Also, the pulse generator mounted in front appears to only have a round two-wire connector, not the rectangular three-wire connector shown in most parts sites.

As before, advice insight and appreciated.

Thanks...Steve.

[jhm5]

Air flow meter working freely? properly?
Air filter good?

John

[Lotsobmws]

AFM moves correctly. No binding and no stripe on the contact patch. Air filter is new.

Still scratching my head...