'94 Sonoma - Low Power / Missing?

Hi All,

I'm hoping that someone can help with the diagnosis of my truck as described below.

1994 Sonoma Extended Cab 4 x 4 - 4.3L (Not High Output)

My truck sat unused a lot this summer. I would started every couple weeks, and maybe take it for a short drive, but only short drives. When I started using it this fall, it was working great for about a week...
The first thing that I noticed was that the Temperature Gage on the dash was jumping around between full pin, and reading correctly (as if it had a loose connection). This also caused the "Check Gages" light to light up on the dash. This didn't effect the way that the truck ran though, and I could tell that the truck wasn't overheating, and it was just the gage acting up.
The next morning (cold morning), when I started the truck, the "Service Engine Soon" dash light was on, as well as the "Check Gages" light and the temperature gage acting up. This time, the truck was running very rough at idle and had very low power. I've driven the truck, despite the dash lights, and noticed that as the RPM's ramp through ~1500 RPM, the engine kicks as if a cylinder is kicking in, but still doesn't have the power that it normally does; as though it's still missing a cylinder.
I took the truck to my local mechanic, and he used his code reader to read the codes from the computer. Apparently there were a few, which he said didn't make any sense. He tried to clear the codes by removing power to the computer, but said that the codes wouldn't clear.
The only other peice of info I can provide is that I stopped for gas at one point and when I restarted the truck, the "Service Engine Soon" dash light was off, and the truck was running great (even though the "Check Gages" light was on and the temperature gage was still at full pin). Shortly after, the "Service Engine Soon" dash light came back on, and the truck started running poorly again.
The "Check Gages" dash light continues to jump on and off, along with the condition of the temperature gage making me think that it is a loose connection somewhere.
Any suggestions?
Many thanks,
Chris

What are the code no?
Is engine missing?
How is the fuel pressure?
TBI injection or multiport fuel injection.
engine code?

Hi,

I did not get the code numbers from the mechanic, but I can check with him and get them again. I do know that the comment from the mechanic was that he didn't think that it was possible that all of these sensor codes would all come up at once. The mechanic felt as though it was either the truck's computer had gone bad, or there was a bad connection to the computer somewhere? Could that be a possible solution?
I'm somewhat guessing to say that the engine is missing, but it is running very rough at idle. The engine RPM drifts up and down while sitting at idle. When I accelerate through ~1500 or ~1600 RPM there is a "jerk" or "burst" of power, but still not as much power as when the truck is running correctly. It's always the same RPM. If I accelerate slowly through ~1500 RPM, the truck will jump and and shake (it feels like a cylinder is kicking in and out) until I'm past 1600 RPM. It's always the same revs though.
I haven't tested the fuel pressure, but I don't believe that it's fuel pump related.
It is throttle body injection.
By engine code, I'm assuming that you mean that it's a Z code engine as designated in my VIN?

Thanks,
Chris

If a lot of codes always clear codes and see what comes back.

If not running good always go over secandary ign for good hot blue spark to all plugs and check for good plugs and wires and dist cap and rotor and base timing.

Hi,

As I mentioned in the original post, my mechanic tried to clear the codes, but said that they all came right back.
Does anyone think that this could most likely be a computer failure?
I'm thinking that it could simply be a bad connector somewhere, but I'm not sure where to start looking. Can anyone offer obvious connectors that I should check?

Thanks again,
Chris

Hi,

As I mentioned in the original post, my mechanic tried to clear the codes, but said that they all came right back.
Does anyone think that this could most likely be a computer failure?
I'm thinking that it could simply be a bad connector somewhere, but I'm not sure where to start looking. Can anyone offer obvious connectors that I should check?

Thanks again,
Chris


What were the code no?

Unfortunately my mechanic did not write down the codes, but from his memory, he recalls them as being:
Throttle Position Sensor Out Of Range
IAC Code?
MAP Sensor Out Of Range

I'm on the verge of buying a new computer for this truck. Does that sound like a drastic step?

Chris

Unfortunately my mechanic did not write down the codes, but from his memory, he recalls them as being:
Throttle Position Sensor Out Of Range
IAC Code?
MAP Sensor Out Of Range

I'm on the verge of buying a new computer for this truck. Does that sound like a drastic step?

Chris


Proper testing before throwing parts at it.
You might give your old pc the tap twist test.
And also go threw the repair flow chart for the codes set.
Good luck

After the tap test and checking ECM connections, you might want to start looking over the engine and chassis grounds.

The ECM sources the engine sensors with its own 5VDC power supply, so you can check that as well. You should be able to read a clean 5V on the supply wire for any sensor, like the gray wire at the TPS or MAP.

Can you expand on what exactly the tap / twist test is?
Should I use a sledge hammer, or a jewelers hammer?
Great tip on the 5 volt supply. I will check that.

Chris

As an additional note, and something that I should have mentioned in my original post: The truck has a bad exhaust leak at the 'Y' pipe near the front of the truck. I'm not sure that this would be the cause of why the truck is running so poorly though?
I just find it so strange that the engine always, always seems to kick, or give a boost of power at exactly the same RPM every time. I think this is a major clue.

Chris

Can you expand on what exactly the tap / twist test is?
Should I use a sledge hammer, or a jewelers hammer?
Great tip on the 5 volt supply. I will check that.

Chris

Use two fingers to sharply tap the case of the ECM - No more. The shock and vibration imparts upon the ECM circuit cards and connections will be similar to what would be transmitted during driving. Of course, if you use a heavy enough "tap" with another tool (similar to my preferred method of adjusting Holley carbs), you'll be certain the ECM is bad:

http://mysite.verizon.net/strey/files/HolleyToolKit.jpg