Help on G30 with 7.4l

[PawPaw271]

I am new here and I need help. I have a 1992 Chevy G30 based small motor home I have had for 20 years with regular maintenance and I have a problem I can't resolve.
The engine starts up ok, but when cold it will hardly run. Low idle, stumbling and no power. Here is a list of what I have done so far.
New plugs gapped at .035, new distributor cap & rotor, new plug wires, new ignition module, idle control, map sensor, vacuum lines, injectors, fuel pump and filter, fuel pressure regulator, oil pressure switch, fuel pump relay, knock sensor, TPS, pulse coil, EGR valve, EGR controller solenoid, ECM and coolant sensor/sender. I also drained all the gas and put in fresh one week ago.
Fuel pressure is at 12 PSI, timing set at 6 degrees BTDC and it is holding a good steady 22 inches of vacuum.
The compression is good and does seem to have jumped timing.
When you first start it up when cold it will never get to the cold start RPMs of about 1200 rpm that was normal for many years. It's only at about 550 rpm until it warms up and then only about 1000 rpms. When it's cold it also seem to be missing. It acts like low fuel pressure, but it shows good on the pressure tester.
One last thing. If I pinch close the fuel return line going to the fuel tank from the back of the TBI with a pair of needle nose the rpms will come up sometimes, like is is a low fuel pressure issue.
This is driving me nuts and I have worked on my own autos for over 50 years.
Any help or insight would be helpful.

Thank you,

Paw Paw

[maxwedge]

Did you replace the 2 wire coolant temp sensor?

[PawPaw271]

Yes the CTS was replaced and tested. I also put on new leads.
One thing I am seeing and I don't understand is the fuel pump set up on these TBI units. I have 12 volts going into the fuel pump relay on the orange wire, but only 9.5 volts coming out on the gray wire. THat is what I am seeing all the way back to the fuel pump with the key on , but not running. I thought it would be 12 volts. I am thinking that that voltage is low and thus could be reducing the fuel pump efficiency or that maybe the fuel pump relay changes the voltage to 12 volts when running. It's a new fuel pump relay also.

Paw Paw

[PawPaw271]

Ok, I think I have finally found the problem, maybe. It seems the knock sensor has become too sensitive. It thinks there is a knock. When this happen the ECU starts knocking timing out of the ignition. Each time it sees or thinks it sees a knock it knock another 4 degrees of timing out and also richens the fuel mixture. It had gotten to the point where the timing was about 20 degrees less than required for good performance.
The result was an over rich mixture and almost no advance of the timing under a load which made it run very poorly and really suck gas.
I am in the process of fixing it now and will post the results.

Paw Paw

[PawPaw271]

Well the knock sensor did not fix it after all. I am now replacing the fuel pump with a higher volume and pressure fuel pump from a 1996 Vortex engine. Delco EP381 pump will now be used. I will know in a week or so if this fixes it as raising the fuel pressure at the TBI did help some. I am now at 15psi, but that is a bit high for the stock OEM fuel pump to handle and it will not last long at that pressure. The new pump will be rated up to 60psi.

Paw Paw

[maxwedge]

Much too much pressure, this will overload the 15psi regulator!

[PawPaw271]

Ok, as a follow up I found the problem. It was the ECM that I had already been replaced. I tried to get a warranty fix, but it was a no go. I then found a used unit in a scrap yard on line and purchased and installed it and the problem has been resolved.
I have taken a couple of trips now and the unit runs perfect again.

Paw Paw