91 Fuel Injection Issue - I think
 

I have a 1991 Chevy C1500 that developed a strong surge in the idle overnight (literally). I ran fine yesterday. It practically will not idle today. It starts OK, but it will run up to about 1500 rpm, then drop down to where it almost dies. Then the cycle repeats - about every three seconds. The condition seems to improve as it warms up, but it doesn't go away. It is not drivable as it is because eventually the idle drops low enough that it dies. It has the 5.7L V8 with throttle body FI. Two weeks ago I cleaned and re-oiled the K&N air filter. It ran fine until today. I've heard/read that the Mass Air Flow sensor can cause this sort of behaviour, but I'm finding that most MAF discussions apply to 1996 and later. Any suggestions? Thanks.

Yeah you dont have a mass air meter on that truck. Its probably the IAC valve or motor. Its what controls the idle when load is either applied or removed to keep a constant smooth idle. Like the A/C or electric loads or even warm up.

Thanks for the tip Ruffneck.
Is there a good way to test the IAC valve, or is it just a matter of taking it out to clean it and then reinstall it to check the performance? Theorectically, since it is just a slave valve of the ECU (or is there a sensor in there too?), if it were removed and the vacuum port sealed, the engine should run at a constant rpm - if the load remains constant. I'll turn off the AC and the like. But if the idle still surges the way it did, I can look elsewhere. Does this sound reasonable?

Of course, if the ECU is looking for a change by reading other sensors in response to whatever it does with the IAC, then this may not work.....

Yeah sensor wise the one that would cause this problem would be the TPS(throttle position sensor).

Well, as most of you probably guessed, my proposed test for the IAC valve didn't work. With the valve removed, I had no way to close the servo loop, so the engine continued to surge about 1000 rpm at idle. Then it would eventually stall. Since the old IAC valve was cruddy I replaced it with the idea that it might be sticking or in some way not responding to the ECM's commands. The new valve had no effect. I was apparentlly barking up the wrong tree.

Instead of throwing more parts at it, I tried a more methodical approach. I removed all of the "rubber" vacuum lines connected to the base of the throttle body and capped off the ports. When the engine was started it stumbled a few times then it assumed a smooth idle and the MIL came on. That told me one of the devices I disconnected was the cause of the surging idle. I then reconnected each vacuum line one at a time until the problem surge reappeared. It turned out to be the MAP sensor was causing the surge by allowing an uncontrolled vacuum leak. Interestingly, it had literally failed over night. This is the first time I have seen such a failure.

I still need to install the new one and verify that all is well, but I'm pretty confident that the MAP sensor is the culprit.

Thanks for your suggestions. They made me look in the right direction.

I spoke too soon. With the new MAP sensor installed, the idle surge is still there. :( It is also becoming very difficult to start. :confused: I also replaced the fuel filter because it was due and I wanted to eliminate it from the equation. As I suspected, the new filter didn't help the problem.

Still, when I remove the vacuum line to the MAP sensor and plug the port it idles very smoothly, though it is difficult to start. As the engine warms up, it begins to stumble a little, but it is certainly not the surge that is present when the MAP sensor is connected.

If anyone can recommend a more logical approach, I'm listening. I haven't messed with the TPS or the EGR valve yet, though I know they can cause similar symptoms. Is there a way these two can be "tested" without actually replacing them? Any suggestions are appreciated. Thanks.

Use a multimeter to test the TPS... Most of the time when the EGR is bad it will throw a code. If you have not already done this, get a can of ether or starter fluid and spray all around the intake, vacum lines, brake booster, fuel lines, with it idling. RPM's will go up if there is vacum leak. Even spray around the throttle body, sometimes the gasket between will leak. The intake manifold gaskets are also known to leak. Sometimes tightening them up will fix it. But new ones are cheap and very easy to put on.

TPS test results
 

Thanks Ruffneck. The TPS sweeps smoothly from 2.1K to 7.1K ohms. Any idea if these values are typical?

The "starter fluid test" will have to wait until tomorrow.....

Well as long as it is smooth. Jumps will indicate bad spots in the TPS. Been thinking bout the MAP sensor. That sensor works with the TPS, coolant sensor, vacum, O2 sensors. It mainly tells the computer what the air/fuel mixture should be according to air density and temperature. So fuel pressure falls into play here. The fuel pump works off of voltage. When more gas is needed, more volts is sent to the pump to increase pressure. So if you haven't already, get a fuel guage and get some readings. Key on, idling, wide open throttle.

This is news to me. Other injection systems I've worked on simply run the pump all the time and allow the pressure regulator to control the fuel pressure. Then the quantity of fuel delivered is controlled by the on-time of the injectors. The pump is only a 12 volt motor with two contacts. Isn't it merely turned on via a relay? If not, what controls the voltage levels applied to the pump? Just trying to understand the whole picture....:confused: