Instant 10psi drop in fuel pressure

[pkufeldt]

Just saw your video, I guess I am chasing my tail with the fuel pressure.

 

[L31MaxExpress]

I was afraid of that. But the larger context is that the engine is misfiring (See https://www.gmt400.com/threads/misfires-return.62862/) and fuel pressure is an issue I can't seem to resolve.

From the service manual:


When it is idling I drop from 51-52 to 49-50psi - I assume that is due to intake vacuum/regulation. I have done a lot to diagnose the misfires and the fuel pressure seems to be the last thing. Here is the reader digest version. Replaced plugs, coil and wires. I did compression and leak down tests (all looks good). I had a bad disty (worn down cam gear), replaced and cam retard adjusted. Had high fuel trims caused by lazy O2 sensors and intake leaks, both fixed and FT are significantly lower. Fuel pressure is the last thing on my list. At a loss.

The MFI Upgrade runs less pressure than the stock spider setup. Never have understood why GM claims they are a direct replacement. My engine ran lean until I swapped the correct injector data into the tune. Could also bandaid the issue without tuning by adjusting the fuel pressure to 66 psi KOEO which is factory spec for the OE style spider. The MFI spider is set at 58 psi. The injector data will still be off but atleast the injectors will flow the right amount of fuel.

 

[pkufeldt]

My engine ran lean until I swapped the correct injector data into the tune.

How do you do that?

 

[Caman96]

How does it run?

 

[L31MaxExpress]

How do you do that?

Tuning the PCM. I stole the injector data out of a 2010 or so 4.3L that uses the upgraded spider as OEM. On 10% ethanol with the wrong injector data my 97 was lean with long term fuel trims approaching +20%.

 

[L31MaxExpress]

As far as misfire goes, have you checked the crank sensor? They have an issue where the reluctor wheel actually rubs on the face and causes them to have an erratic signal. The fix is replacing the sensor and shimming it away with a shim kit.

The distributors also have an endplay issue. Too much endplay causes the shaft and thus CMR and rotor to bounce around. That can cause misfire with maximum timing advance commanded at say 3,000 rpm and no load.

 

[L31MaxExpress]

Here is my post on end play. I have since corrected the CMR by drilling the mounting bolt hole larger in the 4.3L mounting bracket giving me a little bit of rotation to correct the CMR. I set it to the high end of the +/-2° spec at +2° @ 1,000 rpm.

Distributor shaft end play fail!

I noticed my distributor making some weird noises. The rotor tip was contacting the cap electrodes, causing a clicking/tapping sound. I pulled it out and discovered I had 0.055" end play and cracked distributor cap mounting bolt holes despite the thing being aluminum. The gear had some wear...

www.gmt400.com

 

[pkufeldt]

I will look into this.

I got my CMP retard to 0 and it varies from 0.1 to -0.1°. You setting yours to 2 got me worried. Does this look OK?

I don't know exactly what CMP Retard Alternate is, but I watch it anyways. Car Gauge Pro weirdness.

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[Spareparts]

Would be nice to know what CMP Alternate is for sure. Could be a clue to my slight misfire problem.
My CMP also varies slightly with a new gear and cam by about .3 of a degree.

 

[pkufeldt]

No one seems to know what CMP Retard Alternate is. It is not listed in the service manual as a valid PID, but Car Gauge finds it during the scan.