91 Suburban Loping Idle

Help!!!!

My work horse is singing me the blues. It will idle correctly for 1 - 2 sec., then approach death, then iddle correctly again for 1 - 2 seconds. It responds as expected when the throttled of opened for 1 - 2 seconds, any longer it will begin to die.

This is what I have done so far.

1. Changed the fuel filter. Did not check to see of water was in the filter as I only purchase gas from name brand gas stations. Did not think I could get bad gas from them. Was I wrong???

2. Check MAP sensor, voltage signals were close. Changed it out any way. No change.

3. Checked the Engine Coolant Sensor, it is sending the correct voltage.

4. Checked the Idle Air Control Valve. The impedence is as called for in the Chilton repair manual.

5. Checked the Throttle Position Sensor. It appears to be sending the correct voltage as the throttle is opened and closed.

6. Overhauled the Fuel Pressure Regulator. Replaced the spring, diaphragm, and gaskets.

7. Overhauled the Throttle Body. Removed the injector, cleaned all passages with solvent and compressed air. Did not soak the injectors.

In my mind, all that is left is the fuel pump or maybe I did get bad gas. Suggestions ANYONE!!!???

You might check the EGR valve on the intake manifold. It can cause this exact symptom. You will need a new gasket after removing it. They carbon up and stick so have some type of carburetor cleaner on hand after removing it. Also I've seen bad O2 sensors do this kind of problem, on newer vehicles though. '91 computers "think" about as fast as you describe what it's doing.

On throttle body injection. There is a air passage across the back of the unit, towards the firewall. When you removed the throttle body unit, did you clean that passage? If it's plugged, it can cause poor idle problems. A Blazer I worked on would idle ok but die as soon as you put it in gear. It's initial problem was the timing chain was loose and causing all sorts of problems. Sometimes you get two or three systems giving problems at the same time. Hard to troubleshoot when that happends.

Yes, I cleaned every single hole and passage on the TBI.

The egr valve and the oxygen sensor were on my to do list so I went ahead and changed them both out. Unfortunately, no change. I still have the exact same symptons. I checked the fule pump for engagement. I hear a short whir upon turning the key. I depressurized the fuel line and repressurized three times. Every time I could hear the fuel pump make the same whir.

While crawling under the truck I was reminded that the vehicle speed sensor may need to be replaced. Can this be the cause? The speedometer stopped working about a year ago as well as the odometer.

In the mean time I am going to test the remaining sensor (knock, crankcase position, and camshaft position) and the ecm. Don't know if there is a bench test for the ecm. I'll soon find out.

Again, any advice from any one would be appreciated. In any case, I'll keep my thread up to date with the latest info.

Check for vacume leaks as well.

Well, I think I may be on the right path. After examining more electronics, I determined that the knock sensor, electronic spark control my be the cause the problem. I changed the knock sensor, some improvement but I still have a loping idle. The rpm difference is not as extreme. However, it dies when it reaches operating temperature. It does will restart and run with the same loping idle.

I hate throwing parts at a prpblem, but I don't have a scan tool. So unless anyone else has been through this and can give some sage advice, that's all that's left next to bringing it to a shop (I have yet to find one that I can trust). Suggestions....

I'd personaly go back to the idle-air control valve. It's a mechanical device and I'm not sure what impedance would have to do with it if the plunger isn't extending correctly and etc.

For the record, I did install a brand new Idle Air Valve and a new MAP sensor. Neither made any change in performance. I also positioned the pintle within the IAC valve as instructed.

Now for my update...installed a new knock sensor as previously noted. I installed a Electronic Spark Control module that I acquired from a junk yard (would cost $72 otherwise). No change. Leaving me to conclude that I may have a bad computer (still wondering about the fuel pump??).

Vacuum Leaks

I replaced all the vacuum hosed going to and from the throttle body and while ago and have rechecked them several times since. All connections are tight. Where else can a vacuum leak develop that would cause this problem?

Have you checked the catalytic converters? You can check the temp at the pipe inlet and outlet.

RJ now you've got my gears really workin. What can I tell by checking the temp. differential at the converter? What should the differential be? If the diff. is not in spec, what to do then?

I'm not a mechanic and don't believe there is a spec. When my 2001 2500 was having problems holding a steady idle(loping from 50 - 450), my mechanic tested the temp at the converter inlet and outlet. I questioned why and he noted that when the converter material fails, it adds back pressure that is most obvious at idle. Increased pressure from increased rpm's forces the broken material out of the way, or so the explain goes... However, the converter traps heat increasing the temp at the inlet, again so goes the theory...

Have you checked the throttle body gasket??

How many miles? It could be the timing chain, they will cause the loping your talking about as well as poor acceleration. Just a thought.

I did suspect this. I checked the timing with the advance module disconnected. The timing was right on. Is the an accurate check?

Could you do a few tests and let us know what you found?

First, connect a vacuum gage to an unported vacuum scorce on the intake manifold somewhere. Unported means full vacuum is present without opening the throttle.

Also I need to know if chevy put an Opti-Spark ignition on this engine. Does it have a conventional looking distributor at the rear of the intake manifold?

You have to understand there are only a few possibuilities that would cause this. Before throwing parts at it, do a few checks.

Could you do a few tests and let us know what you found?

First, connect a vacuum gage to an unported vacuum scorce on the intake manifold somewhere. Unported means full vacuum is present without opening the throttle.

Also I need to know if chevy put an Opti-Spark ignition on this engine. Does it have a conventional looking distributor at the rear of the intake manifold?

You have to understand there are only a few possibuilities that would cause this. Before throwing parts at it, do a few checks.

No Opti-Spark ignitions ever came on GM trucks/SUVs. The Opti-Spark only came on LT-1 (gen II blocks) 93-97 F-Body, 94-96 B-Body and 92 - 96 (or 97) Y-Body cars.

//2000CAYukon

Could you do a few tests and let us know what you found?

First, connect a vacuum gage to an unported vacuum scorce on the intake manifold somewhere. Unported means full vacuum is present without opening the throttle.

Also I need to know if chevy put an Opti-Spark ignition on this engine. Does it have a conventional looking distributor at the rear of the intake manifold?

You have to understand there are only a few possibuilities that would cause this. Before throwing parts at it, do a few checks.


Test for what? Once connected to an unported vacuum port then what??

Test for vacuum (HG). How many inches of mercury it's pulling while idleing. If vacuum is unsteady or very low. Since the fuel pressure regulator is affected by vacuum, this can cause the engine to have erratic running. If the EGR valve is not closing, this will also show up with a vacuum check. If the PCV valve is sticking open, this will also cause low vacuum. It's just a quick check, nothing complicated here. Kind of helps eleminate some things. A vacuum check can also help detect a flat lobe on the cam if a leakdown check shows good cylinder pressure hold.

Thanks for the info Yukon, I didn't know what all GM used the Opti-Spark on. I'm just trying to help scewturner get this problem solved, it's been quite a while now and he's still battling. An erratic idle problem can either be one of a few things. Those Throttle Body Injectors could drizzle fuel rich/lean caused by a bad coil inside the injector. Fuel pressure is too high caused by low vacuum. I would rule out ignition because the engine idles good for a moment he says. I would put my money on vaccum. Try unplugging the brake booster vacuum hose, I've found power brake diaphragms can go bad and leak. Vacuum hoses collaps inside where you can't see. Squeezing the hose I could feel it was mushy inside. Another oddball thing I found once after an hour was a double-walled exhaust pipe going too the converter had collapsed. The engine would start and run for a moment, then die. This was back in the late '70's and I don't know if GM is still using double walled pipe anylonger. It was a common problem later on with them.

I thank everyone for their input. This is why I like this forum sooooo much. I have finally licked the problem. Turns out it was a rusty fuel tank. I installed a new fuel tank two days ago. Cranked up the truck yesterday and drove it to work today.

Now I must fix the speedometer.

Have 2001 suburban with the 5.3.
Mechanic has replaced EGR valve 3Xs. Error code specifies replace EGR valve. Each time car runs fine for 4-6 weeks (500-1000 miles) and then service engine light comes on. Shortly thereafter the car will not idle correctly (under 1,000 rpm) and will die at stop. Suggestions please.