Advice for tracking down a stumble and misfires

oefvet0708 · Sep 1, 2024

Looking for advice here. At first I was getting a 1345 Ckp to Cmp correlation code. I was able to re-set the distributor forward a tooth and got the timing set at +1*. Light is off now but I still had a terrible cold start issue and a very bad stumble and miss at anything more then 10% throttle and more then 2000 rpms. I tested my fuel pressure and gauge was reading 50 idle and 57 open throttle. Also the pressure would bleed off drastically. So yesterday I replaced the pump and filter and now I’m at 52 psi and 60 open throttle.

Cold start went away entirely and pump on time to prime was drastically reduced. I can get the rpm’s up to about 3000 or so before it starts to hesitate and stumble and probably about 20% throttle but wide open it still falls on its face and starts popping through the intake which sounds like a lean mixture on a carb. So progress but far from perfect.

At this point I’m thinking the spider injection and the fpr is the issue and probably going to need a good tune up. What are some good plugs to use and the proper gap? All of the factory specs are saying .060 but that’s way too much I think. I think something in the .045 range would be better for response. Thank you for the advice.

GreaseMonkey44 · Sep 2, 2024

I run the stock .60, also your distributor would have had to have jumped for you to have to do that? Was it loose when you worked on it? Broken tooth? Makes me think maybe it wasn't needed to be moved if you got the code randomly and nothing changed position wise.
Also you can check for fpr leakage by just signaling the fuel pump relay and looking for leaks with a scope, light a match or take the upper intake off and just look at the spider bare.

Also not that it would make misfires per say but I've seen some of the tps sensors on the throttle bodies twack out so hard that it sets crk and cmp codes on a lot of cars. Had mine act up on me but never set codes just hesitated and randomly bucked harder than hell at certain throttle positions

GreaseMonkey44 · Sep 2, 2024

Just read your other thread about this engine, and answered some of my own questions haha sounds like the distributor is right, +- 0-2 is right on, I'd look for a spider leak or possible vacuum leak

oefvet0708 · Sep 3, 2024

GreaseMonkey44 said:
I run the stock .60, also your distributor would have had to have jumped for you to have to do that? Was it loose when you worked on it? Broken tooth? Makes me think maybe it wasn't needed to be moved if you got the code randomly and nothing changed position wise.
Also you can check for fpr leakage by just signaling the fuel pump relay and looking for leaks with a scope, light a match or take the upper intake off and just look at the spider bare.

Also not that it would make misfires per se but I've seen some of the tps sensors on the throttle bodies twack out so hard that it sets crk and cmp codes on a lot of cars. Had mine act up on me but never set codes just hesitated and randomly bucked harder than hell at certain throttle positions

I bought the truck recently in this condition and knocking one thing after another out. I had initially the relearn for the crank position but it didn’t help. But after I did the relearn and the code came right back I looked at the Cmp retard pid and it showed -24* so I looked at some other threads and came up with that solution and it worked to clear that code but still getting misfires. I think I’ll drive it tomorrow morning to work and do a data log and see what’s up.

Schurkey · Sep 3, 2024

oefvet0708 said:
yesterday I replaced the pump and filter and now I’m at 52 psi and 60 open throttle.

Did you replace the fuel filter?

oefvet0708 said:
I can get the rpm’s up to about 3000 or so before it starts to hesitate and stumble and probably about 20% throttle but wide open it still falls on its face and starts popping through the intake which sounds like a lean mixture on a carb.

What is the fuel pressure when it's "falling on it's face" and popping?

oefvet0708 said:
I’m thinking the spider injection and the fpr is the issue and probably going to need a good tune up.

Seems reasonable, but I'd connect a scan tool and verify EVERY sensor and computer output first.

How old are the O2 sensors?

Will the ignition coil reliably fire a spark-tester calibrated for HEI? (Preferably with the coil fully warm from ~20 minutes of driving around, and with the top of the coil misted with water from a squirt-bottle.)

oefvet0708 said:
What are some good plugs to use and the proper gap? All of the factory specs are saying .060 but that’s way too much I think. I think something in the .045 range would be better for response.

The stock, GM-recommended plugs should be fine. However, like you I also prefer a smaller gap of .040--.045. This will require a different plug with a shorter ground strap--don't bend the ground strap of the wide-gap plugs.

In fact, don't re-gap any fine-wire spark plugs--Platinum, Double-Platinum, Iridium, Kryptonite, etc.

oefvet0708 · Sep 3, 2024

Schurkey said:
Did you replace the fuel filter?

Yes

Schurkey said:
What is the fuel pressure when it's "falling on it's face" and popping?

oefvet0708 said:
yesterday I replaced the pump and filter and now I’m at 52 psi and 60 open throttle.

60

Schurkey said:
How old are the O2 sensors?

Less than 3 months. Previous owner replaced them as part of his trouble shooting this issue before selling it. I verified the condition of them when I replaced the fuel pump and filter over the weekend

Schurkey said:
verify EVERY sensor and computer output first.

yes I am going to do a data log tonight or in the morning to verify sensor operations. I’m concerned about the maf. Previous owner put spade connectors in the maf harness. Could be skewing the signal form higher resistance.

I do not have an hei spark tester

oefvet0708 · Sep 3, 2024

Ya

GreaseMonkey44 said:
Just read your other thread about this engine, and answered some of my own questions haha sounds like the distributor is right, +- 0-2 is right on, I'd look for a spider leak or possible vacuum leak

yes I thought about doing a simple vacuum test that sounds have some indication of the overall health of the engine. Also considering a cylinder compression test to see if any of the valves are messed up. If anything it’s a good idea of how healthy everything is

Schurkey · Sep 3, 2024

oefvet0708 said:
I do not have an hei spark tester

www.amazon.com/dp/B003WZXAWK/?

oefvet0708 · Sep 4, 2024

Schurkey said:
www.amazon.com/dp/B003WZXAWK/?

Ordered. It won’t be here until the 11th. I tried looking that part number on Orielly but couldn’t find it

oefvet0708 · Sep 4, 2024

I am also going to try another fuel pressure gauge. I think the one I had used initially was not getting a good reading and I tried looking at pressure again tonight and got three different readings and would not read prime pressure. A friend is going to bring his kit to work in the morning

Advice for tracking down a stumble and misfires

oefvet0708

Newbie

GreaseMonkey44

Newbie

GreaseMonkey44

Newbie

oefvet0708

Newbie

Schurkey

Supporting Member

oefvet0708

Newbie

oefvet0708

Newbie

Schurkey

Supporting Member

oefvet0708

Newbie

oefvet0708

Newbie

Similar threads

Latest posts

Members online

Forum statistics

Share this page