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:

The computer controls how much voltage is sent. Just like the e-fans. When the temp gets too high, the computer will send more voltage to the e-fans to draw more cfm. This is how it is on the Gen III motors. What you have may be different so Im not real sure. The regualtor works off vacum. At wide open throttle the regualtor has no vacum applied so it is free flow. Thats why some guys will disconnect it to get free flow all the time at the track.

Thanks Ruffneck. That helped to fill in some of the blank spots in my mind. (plenty of those :o)

What constitutes a Gen III motor? 1996 and newer?

The 99-06 vortec motors. The Gen IV started with 07 till present. The ignition set up is different from what you have. No distributor, it uses crank and cam sensors, individual cylinder coils, MASS sensor, heated O2 sensors, computer has more control over all. So they are more efficient and precise as for like air/fuel ratio and timing.

"Starter Fluid" test
 

Again, thanks for the education. It sounds like the Gen IV engines have systems I'm more familiar with from working on my bikes. They don't have the MASS or O2 sensors, but the other elements are present, and the bikes are ten years old.

I had a chance to spray some starter fluid around the TB and such today. I had to run it with the MAP sensor disconnected and capped because that is the only way it will idle - around 900 to 1000 rpm. The only "sensitive" area I could find was when I sprayed on the EGR valve. However, the RPMs did not increase, but rather, after 2 or 3 seconds the rpms dropped signifcantly and a couple times nearly stalled if I hadn't given it more fuel. I did this repeatedly to make sure it wasn't just a fluke. Every time the rpms would drop.

Now, my question is, does this indicate a flaky EGR valve or am I merely introducing a condition that would affect a fully functional engine the same way? I know the EGR valve is supposed to recirculate exhaust gases into the intake, but is it also normally open to the "outside world?" I'm just trying to avoid needlessly buying yet another part, only to have someone tell me later that, "Yeah, when you spray starter fluid on your EGR valve it will make your engine stall. I thought everyone knew that?" :rolleyes:

I'm not cheap, but I would just rather spend my money on fixing the problem - not replacing a serviceable part, as I have done three times already up to this point. :(

Let me know what you think. Thanks for your input.

Ok when the EGR valve starts to become faulty, it builds up carbon. Therefore it cannot close or open fully. When it gets stuck open, it will cause the motor to run sluggish. You will also see some black smoke from the exhaust. Sometimes it will not throw a code for the EGR if it stays open, but a rich code. If you rev it up a lot check your exhaust for excessive black smoke. If it stays closed then nothin will really happen except a EGR code. On the other hand faulty O2 sensors can cause this too. On my 2001 I can take the EGR off and clean it. A lot of folks do that for maintainance. You will need a new EGR gasket too. First, have you gone to Autozone and had it checked for codes? The computer will store codes and run default settings until the problem is fixed. Do that first if you have not already. Even if the CEL has not come on.

I replaced the O2 sensor just before the MAP sensor because it was relatively cheap and I have heard they can fail and cause symptoms similar to mine. The new one made no improvement.

I checked the codes myself per the Haynes manual I have been using. It only flashes the "12" code (continually) which Haynes says merely indicates that it is in the diagnostic mode. Whereas, if a fault code were stored, it should flash "12" three times and then flash any stored codes in succession - each one three times. Also, the only time the "check engine" light comes on is when I have the MAP sensor disconnected.

During the weeks leading up to this failure - even the day before- it ran fine - not sluggish at all. I've never seen any black smoke from the exhaust, even during the rare times that I ride it hard. The tail pipe has some "color" on it, but not what I would call sooty or carboned from a rich mix.

When I was testing it with the starter fluid and had to rev the engine to keep it running, I could see the EGR shaft/diaphram actually moving. However, it didn't travel very much. It looked like maybe a 1/16 or 1/8 inch. I'm not sure what is typical movement on those valves, but it was doing something. If nothing else, I guess I'll have to pull it off to inspect it more closely. If I apply my own vacuum to it, do you have any idea how far it should be moving? When it is closed does it rest against a seat in the intake manifold? The valve operation will probably be obvious once I get it out to look at it.

Im really leaning more towards a vacum leak. Even a bad PCV valve could cause this. Take your throttle body off and look all around. Even those flaps sometimes do not close all the way and more air enters thus causing the computer to try to keep a constant idle but becomes more of a surge.

Those flaps?? I'm not following you on this one.......
Pull the TB, eh? That will be a project for Monday. I'm booked for tomorrow, :) but maybe I can pick up the gasket kit.

The PCV valve. That's certainly a cheap part I can eliminate. Although I didn't suspect it because when I had it disconnect and capped, and then reintroduced it to the system, nothing changed - at least nothing apparent.

Another question, though likely unrelated: The PCV valve is in the right hand valvel cover, but on the left hand valve cover another tube (about 1/2" diameter) comes up and enters just above the TB but below the air filter. Is that simply a crankcase breather similar to what was installed on older GM vehicles with that small filter inside the air filter canister?

I appreciate your time and effort Ruffneck.

Hey no problem. That tube is mainly for vapors that rise from within the motor from heat. Even though there is no water inside, heat will cause some condensation from the oil and internal moving parts. This creates vapor and will rise. So it is set up to be vacumed and thrown into the intake. If not then that condensation will settle, which will create water within the motor. The flaps Im referring to are the ones that open when throttle is applied to allow more in. They sit just below where the injectors spray in the throttle body. When some gunk builds up inside the throttle body, mainly from that hose and PCV valve line, it will sometimes not allow the flaps to fully close therefore letting more air in and throwing the TPS off. Most of the time a can of Throttle Body Cleaner sparayed into really well the throttle body will do the job. But taking it off and cleaning by hand with a brush and cleaner is better. You can inspect without taking it off. Just take the air cleaner off and look down inside the throttle body(with the motor off), and open the flaps by hand. If its dirty you will see black sludge build up.

OK, I got distracted with another project. Sorry for the interruption.

Now that I'm again investigating my FI problem, I have satisfied myself that the issue is not the MAP sensor. At the MAP sensor connector, the +5 volts and Ground are present, and when I apply vacuum to the sensor the center output contact swings from +4.6 volts to +0.7 volts. I also traced the continuity of the sensor's output conductor back to the ECU. It was good and didn't show any intermittent behavior.

Now, since the general opinion is leaning toward a vacuum leak, is there any benefit in measuring the manifold vacuum? If I had a vacuum gage, are there any meaningful readings I can take that might point me in the right direction? Where would I connect the gage? What values should I expect to see?

The PCV valve has been eliminated, so it is not the source of a leak.

I'm resurrecting this thread merely to bring it to a logical conclusion. For those of you who had the interest (or fortitude) to read through it, you'll be happy to know that the root cause of my idle problems was a tired fuel pump. For those of you who helped me figure that out, thank you very much. :D

Actually, I owe Ruffneck an apology of sorts because he suggested early on to check the fuel pressure, but I figured since the engine would start and run (though with difficulty) that it couldn't be the fuel pump. I could SEE that the engine was getting fuel. True it was getting fuel, but the pressure was too low. Well, I have now been educated thanks to you. Had I listened to you then, I could have saved myself much time and money.:o

In a nut shell then, if your truck begins to suddenly idle strange and it won't take throttle, be sure to verify that the fuel pressure is correct before you waste a lot of time and $$$ looking elsewhere. :cool:

Cheers