1994 1500 hard starting, stumble off idle, hesitation at light throttle

[L98camino]

Hello everyone, long time lurker, first time posting.

I am borrowing my buddies '94 1/2 ton Silverado, 5.7L auto 2wd. It has about 180,000 miles on it and he is the original owner. I've been doing the maintenance on it for a few years so I know what has and hasn't been replaced recently.

As of today I have changed plugs, wires, cap and rotor, TPS, fuel filter, oil pressure switch and fuel pump relay and fuel pressure regulator spring. I replaced the EGR valve (non oem part, I didn't know then to stay away), it is currently disconnected and vac line plugged, it runs worse with it hooked up. The fuel pressure is rock solid at 12.5 psi. I seafoamed the intake with no change. The check engine light isn't on but there is a code 33 stored.

It has the dreaded off idle stumble, and hesitation at cruise with light throttle. It runs smooth and strong otherwise. It also has been progressively getting harder to start, this has me the most concerned right now. When cranking it acts like the timing is off, starter lurching and stopping then running up again. I attempted to check the timing but when I disconnect the timing set connector it acts like the timing is WAY off and won't stay running. I advanced and retarded the timing and it made a difference in throttle response, for the worse either way, so I put it back where it was. Not sure where it is set at because the mark is about an inch from the end of the tab in the advanced direction with the set connector hooked up (that is where it was when I started).

I have read all the stickies and I am at my whitt's end. My '67 C10 is in a million pieces so I really need this truck.

Please help or let me know if I should wash it and give it back to my friend, LOL.

Thanks in advance, Dan

[speedygonzales]

Code 33 is for the MAP sensor circuit. Should pay attention to that problem first and foremost. Sensor codes should be dealt with before you do anything else. MAP sensors are bullet proof so don't just replace the sensor. Do a little diggin......

[L98camino]

Thanks for the quick response Speedy!

Not sure what to dig for, I checked for leaks in the vacuum lines, the connector and harness look to be in good shape. How do I test it for proper function?

Thanks again, Dan

[speedygonzales]

Looking at electrical connections is just a small part of the investigation.

First when dealing with a sensor failure, reset the code and see if it returns. If it does

You have to check the presence of voltage, ground and signal. I want to say that I do NOT have a 1994 manual but most MAPS will work in the same manner.

You look for the presence of +5V and Gnd. I do not know what your wire colors are so lets assume for the moment, it's the same as mine. The black wire will be ground and the Grey wire is +5V. So between those 2 wires, you should measure 5Volts.

Without an active scanner, you really can't test the device so in the place of the scanner, you check the connection from the device to the ECM. If you have 5 volts, ground and the signal wire connects all the way back to the ECM you really have no choice but to replace the sensor. Although as I said it is usually bullet proof.

[L98camino]

Thanks speedy.

I pulled the ecm fuse and reset the computer. Drove it around for an hour, stopping now and then to get the comp to relearn. Code 33 did not come back.. I think the code was set when I unplugged the timing set connector..

Has anyone had the same problem when the timing set connector is disconnected?

[ChevyTech]

People have a lot of trouble reading codes by counting flashes, so maybe it had a code 42.

Unplugging the wire to set the timing sets a code 42.

[L98camino]

Thanks for the input Chevy Tech.

I have only had one code come up, I am sure it was 33. I reset the ecm and in 2 days of normal driving it hasn't come back.

It never set a 42 code when I disconected the timing set connector, but the truck would not stay running. It acted like it was WAY out of time.

I put the light on it and it was way advanced (an inch away from the timing tab in the advance direction) with the timing set connector connected.

Is this normal? Why does it buck and die when I disconnect the connector?
Thanks, Dan

[ChevyTech]

It is normal to see the timing at 12 to 20 degrees advanced at idle when the set timing connector is plugged back in.

Many TBI trucks stall when the set timing connector is unplugged. Make sure you have the engine at normal operating temperature and all accessories off when trying to set the timing.

If everything is in good condition they have a slow poor idle when the set timing connector is unplugged.

If you got the 12.5 PSI fuel pressure with the engine runiing that is good. I would look at the distributor next.

On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

If there is oil coming up the shaft and getting out onto the pickup coil – replace the distributor.

These distributors can also have problems because of loss of magnetism needed for the pickup coil to work.
If the magnetic material on the pick-up coil is cracked into pieces replace the pick-up coil.

[L98camino]

Thanks Chevy Tech!

I will dig into the distributor tonight.

Dan

[L98camino]

Sorry I havent been back for a while, but we fixed the truck and I wanted to make sure before I posted.

Thanks to Speedy Gonzales and Chevy Tech for all the great advice.

Chevy tech, you were spot on with the distributor pick-up magnet. It was deteriorated, cracked and falling apart.

New dist. and it runs better than ever.

Thanks again to everyone who chimed in with suggestions and advice!

Dan

[ChevyTech]

You're welcome Dan, and thank you for letting us know what fixed it.

On the these TBI trucks the magnet that induces a voltage on the pickup coil is actually part of the distributor shaft. Gm does sell the shaft by itself but I got sticker shock when a friend of mine told me what he paid for a shaft at the local dealer.

Many of the various brands of aftermarket distributors are having modules failure very soon after they are installed. If you have any problems soon, have the new module tested.

Many auto parts stores in my area will test the distributor module for free if you bring it in.

Make sure when you install or reinstall the distributor module that you use heat sink compound under it. The compound helps dissipate heat from the module. Without it, the module will fail.

Glad you got it fixed.

[Kudzupatch]

Originally Posted by ChevyTech

..... Many of the various brands of aftermarket distributors are having modules failure very soon after they are installed. If you have any problems soon, have the new module tested.

I can second this! Just recently had that happen with a reman. distributor and went back with a GM module.