Rough running after operating temperature is achieved

DAVID SWORD SR

MEMBER
1998 CHEVROLET 1500
4.3L
6 CYL
2WD
MANUAL
203,000 MILES

When engine is cold, truck starts right up and runs perfectly. Will continue to run perfect (for as long as engine is running) until shut off. After shut off (for about 10 minutes) upon restarting engine runs horrible! Have changed, spark plugs/wires, cap/rotor, TPS, ICV, coolant temp sensor, fuel filter, air filter, oil/filter, EGR valve/upper radiator hose, crankshaft position sensor and fuel pump. I am totally stumped!

8 Replies

DAVID SWORD SR

MEMBER
6 POSTS

I will add that the "spider assembly (including fuel pressure regulator)" were replaced approximately 3 months ago. This problem has been occurring for approximately 2 weeks. Also, removed CAT(s) 7 years ago when it became legal to do so and installed "true dual" exhaust.

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Tuesday, September 29th, 2020 AT 7:02 AM

ASEMASTER6371

MECHANIC
52,797 POSTS

Good morning,

I would start with the fuel pressure. Check it and make sure the pressure is correct.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

You need 55-60 pounds of pressure.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

The issue is the fuel pressure regulator. It is common to fail and when it does it drips fuel directly into the intake manifold and makes it run rich and also lowers the fuel pressure as well.

Are there any codes set?

If you have a scan tool, can you give me the long and short term fuel trim percentages?

Roy

REMOVAL PROCEDURE
1. Disconnect the negative battery cable.
2. Relieve fuel system pressure. Refer to Fuel Pressure Release Procedure.
3. Remove the upper manifold assembly. Refer to Engine Mechanical.

DISASSEMBLE PROCEDURE
1. Disassemble the fuel pressure regulator snap ring retainer.
2. Disassemble the regulator assembly and the O-rings.
3. Discard the O-rings.

NOTE: If the fuel pressure regulator is to be reinstalled, inspect the filter screen (if equipped) for contamination. If contaminated, remove. Discard the filter screen.

ASSEMBLE PROCEDURE
1. Lubricate the new O-rings with clean engine oil.
2. Install as an assembly, backup O-ring, large O-ring, filter and small O-ring.
3. Assemble the regulator with vacuum tube pointing down and snap ring retainer.

INSTALLATION PROCEDURE
1. Install the upper manifold assembly. Refer to Engine Mechanical.
2. Connect the negative battery cable.

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Tuesday, September 29th, 2020 AT 8:04 AM

DAVID SWORD SR

MEMBER
6 POSTS

I installed a new fuel pump yesterday. Fuel pressure is still a bit low. 52 lbs. With key forward (not running). After start up, drops to 45. After shut off, shoots up to 55 and holds, decreasing very slowly over an hour. I thought the "holding of fuel pressure" was a good sign it was not the pressure regulator. But, I'm not an expert.

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Tuesday, September 29th, 2020 AT 10:09 AM

ASEMASTER6371

MECHANIC
52,797 POSTS

I would start with the regulator as I suggested. It is part of the spider but can be replaced by itself as in the picture I gave you.

Roy

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Tuesday, September 29th, 2020 AT 10:30 AM

DAVID SWORD SR

MEMBER
6 POSTS

After further inspection, (although it's right in front of me and several mechanics) the "heat sink" is missing from my ICM. I'm going to try a new ICM and new heat sink before I tear the intake manifold apart.

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