[SOLVED] 1995 k2500 7.4 TBI running HORRIBLY

[Cuckfield]

Hello, I have the above mentioned configuration.
I did a tune up a week ago (cap, rotor, plugs, wires, CTS) to address what I thought was a bad ignition set, because I have noticed when my truck gets warm it runs much worse and shudders on acceleration and misfires.

No improvement after these replacements; I have fully verified the job was done properly, and timing was dead on at 4 degrees when checked with a light.

However, today, my truck has been CONSTANTLY misfiring, from cold start through to warm. It also idles incredibly badly, sounds like it wants to die, and is completely gutless. It surges and bogs, and to get anywhere in it I have to gently feather the throttle.

Not only that, but when I am at high speeds (50+) and I take my foot off the gas, it backfires, loudly. Like a gunshot going off.

The fuel pump makes noise when I prime it, the whirring noise that is normal. I replaced the filter and tested pressure with a loaned autozone gauge and I got like 20+ psi. Not sure what that's about.

I am at a loss for what is next. I have trawled the forums and I frankly don't know what to check next. What could possibly cause it to suddenly run THIS badly? It's practically unrivaled, but by some miracle it doesn't die.

 

[EJ_74]

Have you checked for vacuum leaks?

 

[Cuckfield]

Have you checked for vacuum leaks?

I have not. What is the quickest and most expedient way to do so?
Also, would a video of what it sounds like idling help? It makes quite an interesting racket. I limped it to work today and my coworkers were standing outside wide-eyed in horror as I rolled in!

 

[Schurkey]

timing was dead on at 4 degrees when checked with a light.

Did you disconnect the timing connector before verifying the timing with the light?

Did you have to turn the distributor to get the timing correct?

The fuel pump makes noise when I prime it, the whirring noise that is normal. I replaced the filter and tested pressure with a loaned autozone gauge and I got like 20+ psi.

A '95 454 is a higher-pressure TBI system. I'm thinking 20-ish psi is low. Would be good to verify with the service manual set for your vehicle that you should download from the links in the Sticky thread section of the Engine forum.

Connect a scan tool, find out what the computer wants you to know. "Codes" can be useful, the real diagnostic power is in the data stream--sensor and output device data--and the bi-directional control.

 

[GoToGuy]

CTS - coolant temp sensor? Which one ? The one in the head head? Or the one for the ECM with two wires? Read this page. √

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[someotherguy]

A '95 454 is a higher-pressure TBI system. I'm thinking 20-ish psi is low. Would be good to verify with the service manual set for your vehicle that you should download from the links in the Sticky thread section of the Engine forum.

26-32 psi on the 1994-1995 454 higher pressure system

Richard

 

[Cuckfield]

Did you disconnect the timing connector before verifying the timing with the light?

Did you have to turn the distributor to get the timing correct?


A '95 454 is a higher-pressure TBI system. I'm thinking 20-ish psi is low. Would be good to verify with the service manual set for your vehicle that you should download from the links in the Sticky thread section of the Engine forum.

Connect a scan tool, find out what the computer wants you to know. "Codes" can be useful, the real diagnostic power is in the data stream--sensor and output device data--and the bi-directional control.

Yes, I disconnected the vacuum advance.
No, I did not have to turn the dizzy.
Can you read a '95 with an obd1? I always hear you can't.
I will double check and re-verify fuel pressure tomorrow.

CTS - coolant temp sensor? Which one ? The one in the head head? Or the one for the ECM with two wires? Read this page. √

You must be registered for see images attach

The one at the front of the engine in the water jacket. Not the one on the driver side between the spark plugs.

 

[someotherguy]

There's no vacuum advance on these trucks. The timing bypass wire which is tan with a black stripe, has a disconnect in various places depending on year (TBI engines only) - you'll likely find it for your 1995 model in the wiring loom under the glovebox.

I assume you did find this though, if you saw 4° instead of a greater number that bounces around a lot which would be the behavior expected with the bypass wire still connected.

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Richard

 

[Cuckfield]

There's no vacuum advance on these trucks. The timing bypass wire which is tan with a black stripe, has a disconnect in various places depending on year (TBI engines only) - you'll likely find it for your 1995 model in the wiring loom under the glovebox.

I assume you did find this though, if you saw 4° instead of a greater number that bounces around a lot which would be the behavior expected with the bypass wire still connected.

You must be registered for see images attach

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Richard

Vacuum advance, timing advance, oops.
Yeah, this is what I unplugged.

 

[DerekTheGreat]

Fuel pressure, resolve that. As Schurkey and someotherguy pointed out, ~20psi is too low.

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