Surging idle issue and bad fuel economy?

[91z71]

Okay, I have a 1991 GMC k1500 with the 350 T.B.I and it has had a pretty bad surging idle for a few years now. It hasn't given us any real issue really, but its really annoying. It only does it when it is warm. If it is cold, the surging isn't there. As it warms up and gets closer to tempature, it gets worse and worse. It Fluctuates between 500-900rpms. My dad thinks it may be the fuel pump, but no real test has been done to confirm it.

The wires look brand spanking new and there is no tears, the boots look good and there is no cracking, and the wires are really flexible. I'm failry certain the previous owner replaced them. The spark plugs were changed out and are only a few months. They look like they are supposed to. It doesn't burn any oil, and I noticed the other day when I filled up the tank, im getting 8.5mpg... That can't be right, maybe I have the tank size wrong? I'm getting about 200-210 miles between fill ups. Is that normal?

[special-K]

Can you feel anything when you grab the wires with the engine running? Best way to know how the plug wires are is look at night for arcs. Sprinkle a little water to get a real good idea. But, I kinda doubt that's what it is. I'm no tech but it's probably an idle sensor or whatever is involved in "reading and sending" settings

[91z71]

Figured out the 8.5MPG thing.... my truck leaked 1/8 of a gollon of gas in the parking lot today when I turned it on to let it warm up. The fuel filter was a little loose and gas was spraying everywhere. I tightened that back up and made sure it won't come back off again, it stopped the surging idle, but now I can hear a notciable miss when its idling? While driving it doesn't do it. Itll idle fine, but every now and then the truck shakes and I hear a different note in the exhaust. The spark plugs look fine, and there is no indication on which cyclinder is missing? I am at a loss here, the plugs are also new, so I assume the miss has something to do with the ignition system?

[speedygonzales]

When was the last time you looked inside the cap and at the rotor?

[Kudzupatch]

I agree (for what that is worth). And I have recently been thought this and my problems were multiple but all electrical. Wires can be bad even thought they are new. Ignitions modules act up but still work. New one fail in months. Plugs can be bad and look good.

[ChevyTech]

Check it for trouble codes, which can be done without a scan tool on your 1991.
http://67-72chevytrucks.com/vboard/s...5&postcount=11


Test the fuel pressure with the engine running.
http://67-72chevytrucks.com/vboard/s...d.php?t=375132

Check for vacuum leaks. A common spot is the base gasket under the TBI unit.

[91z71]

Okay, my battery died yesterday because I left the dome light on my mistake and I noticed something... there is a pile of coolant on top of the motor its coming out of the upper radiator hose... We never noticed it because it gets cooked off when the motor is hot.... but it sat for a day or two and there it was. Would that cause a vaccum leak??


I guess that would explain why we would slowly loose coolant and have to top it off everynow and then. lol

[91z71]

On further inspection it looks like its leaking from the fitting that the hose clamps to. I don't know exactly what it is called, but it has two bolts holding it on.

[ChevyTech]

Originally Posted by 91z71

On further inspection it looks like its leaking from the fitting that the hose clamps to. I don't know exactly what it is called, but it has two bolts holding it on.

Thermostat housing!

[91z71]

Originally Posted by ChevyTech

Thermostat housing!

Could a leaking thermostat housing cause a rough idle/missing issue? It only does it when the truck his operating tempature. When the truck was warming up, it doesn't do it.

BTW, thanks guys for putting up with my lack of understanding of cars and helping me out. This is the first time I have taken initiative to figure out and issue on my own. Usually my dad does all the diagnosing and I just hand him parts/tools as he asks for them, so I havn't learned much unfortunately.

[ChevyTech]

The coolant sensor that the ECM monitors is right next to the thermostat. If the coolant leaks bad enough that the coolant sensor is in air rather then coolant it could cause issues.

It sounds to me like it runs ok in “Open Loop operation” when the ECM injects a pre determined amount of fuel using only some of the sensors.

When the engine is cold the ECM does not use the oxygen sensor to determine how much fuel to inject.

Air getting in the exhaust system can affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.

Does the truck have exhaust leaks?

[91z71]

Originally Posted by ChevyTech

The coolant sensor that the ECM monitors is right next to the thermostat. If the coolant leaks bad enough that the coolant sensor is in air rather then coolant it could cause issues.

It sounds to me like it runs ok in “Open Loop operation” when the ECM injects a pre determined amount of fuel using only some of the sensors.

When the engine is cold the ECM does not use the oxygen sensor to determine how much fuel to inject.

Air getting in the exhaust system can affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.

Does the truck have exhaust leaks?

Yes, on the muffler. It is on the output side of things, so my dad told me not to worry about it. It is about the size of a quarter, it looks like it was patched by a previous owner and the patch came off.

We still havn't fixed the coolant leak, we just make sure it is topped off every now and then. We are in the process of moving, so my dad doesn't want to mess with any of the cars/trucks till we are settled in.

[OutlawDrifter]

i would give it a full tune-up before you do anything else. plugs, wire, cap, rotor, also check the distributor to see if the shaft has any play in it....it may need to be replaced also.