Cartrax

Trouble shooting misfiring under load, currently looking at FP readings, observed the following... (Have VRFPR set at 23 PSI, 383 stroker, EBL, parallel plumbed 80# Injectors, bored TB's, Renegade, more... )

...with vacuum line attached to regulator - at idle it jitters between 12 & 15 PSI - snap the throttle and it jumps to 20 - hold at about 1/4 throttle for 5 seconds or so and FP drops to around 13 PSI and runs rough.

Same three tests with vacuum removed from regulator and it holds 23 PSI across the board. Also it runs a bit less rough at 1/4 throttle but not anywhere close to smooth. Concluded fuel pump is probably OK.

Does it look like a vacuum line issue? Regulator vacuum line is attached to passenger side TBI far right port. Should I relocate it??

Schurkey

Why is the idle vacuum unsteady?

Agree. This is not a pump problem. This is a tuning problem, or you have mechanical issues like broken valve spring, leaking valve(s), vacuum leak(s), etc.

I would start with a cranking compression test. Depending on what the results are, you might want to add a leakdown test, and/or a cylinder-balance test. I love to see the primary and secondary ignition pattern for each cylinder on an oscilloscope, but not many folks have access to an automotive 'scope. A vacuum gauge might tell an interesting tale, too.

Would be worth your time to check the data stream, to see what is happening during the "rough running".

Cartrax

Thanks, yep jittery gauge could be a valve issue - damn. Vacuum test coming. Recently rebuilt block hoping it's none of the serious mechanical issues you mentioned. No oscilloscope. Have compression testor...

Cartrax

Quoting myself ... anyway got a vacuum reading it doesn't jitter it drifts back and forth slowly. Could be the CAM or idle control valves, or another issue but not the valves, thankfully.

Schurkey

Does the speed of the vacuum drift match the rise 'n' fall of the fuel pressure? I'd want to know if they're correlated.

Cartrax

No, jitter in FP occurs with or without vacuum line attached to VRFPR. Cadence is completely different. Vacuum gauge drifts slowly and inconsistently. FP is a constant jitter of about 4 PSI.

Schurkey

I've never worked on CFI, but I wouldn't expect 4 psi "jitter" in fuel pressure to be normal. Defective regulator, defective pump, failed or missing pulse damper...I think it could be any one of several things, or even a combination. "I" would put a current (low-amperage) probe on the fuel pump + wire, and watch the results on a 'scope. The current draw of each armature bar should be equal, or nearly so. But if you don't have access to the probe or the 'scope, it's going to be a lot harder to determine if the pump is having issues.


Manifold vacuum variations are almost certainly unrelated. Carb'd vehicles can cycle manifold vacuum when the idle mixture screws are set too lean.


How old is the O2 sensor? In fact, what is the data stream showing?

Cartrax

Thanks for reply... O2 sensor has less than a couple thousand miles on it. No scope but thinking of purchasing something like the Hantek 1008C PC USB 8CH as this is second response that recommended the use of an oscilloscope. I could use something that will help isolate where the issue(s) are - ignition vs fuel delivery or whatever.

Would the Hantec or similar do the trick? Has decent reviews...

By "data stream" I assume you are referring to the EBL Data Log?

Schurkey

I have an ancient Snap-On, and a hand-held Matco/OTC. Both of mine are 2-channel. 8-channel sounds like overkill. 4-channel might be the "sweet spot". As you use more channels, the 'scope slows down--it can only process so much information in a given time. There's lots of 'scope discussion here:

https://www.garagejournal.com/forum/...t=oscilloscope


If you buy the Hantek, you'll still need amperage probes, ignition probes, hopefully a vacuum/pressure probe or two, etc. The price on the accessory probes adds up!



I don't know EBL, either. The Data Log may be the equivalent of the data stream--a readout of all the sensor signals and actuator commands that the computer is processing, in more-or-less real-time.