Rough Idle

[Tominator]

Earlier I posted about an ECM problem about my 88 Chevy pickup with the 5.7L motor. Before with the EST wire unplugged It was idling great, so that lead me to the conclusion that it was the ECM. I replaced the ECM plugged the EST wire back in and it idled great. Now all of a sudden I'm back to a rough idle again. I'm not getting any more trouble codes so all seems well electronicaly.

When I rev the motor and hold it at a higher RPM it smooths out.
Would this seem to be a fuel pressure problem?

If so what is the best way to cure this. Seem like an external pump would be the way to go.

Thanks for any help.

[KA9MOT]

Fuel, Elctrical problems or a vacuum leak.

[Tominator]

Thanks Beef,

That helps me narrow it down!

[TNcarguy]

This may be simple and obvious, but I'll ask anyway. Have you changed that fuel filter lately?

[OffRoader]

Originally posted by TNcarguy
This may be simple and obvious, but I'll ask anyway. Have you changed that fuel filter lately?

I would also suggest fuel filter. However it has been changed recently, then vacuum leak would be my call. Vac leaks are, IMO, one of, if not the most overlooked problems. Cracked hoses, bad PCV valves all easy things to over look. People start swapping out this expensive sensor, or that costly valve, all the while a 10 cent rubber hose is the cause. JMHO.

[Tominator]

Thanks off roader, I had checked for vacuum leaks by spraying carburator cleaner all over the place. I know ether works better but I don't have any on hand.

It's getting to the point where it barely runs so I'm starting to think along the lines of fuel delivery (fuel pressure, injectors).
It does have a new fuel filter in it. Still not getting anymore trouble codes. I think that I'm a victim of having two major problems at the same time making it a bear to troubleshoot.

I guess I'm going to spring for a fuel pressure gauge and check that. The truck has 125K mi. on it so I'm guessing that it's possible the the fuel pump could be going south on it.

[OffRoader]

If you don't want to own a fp gauge, just rent one from AutoZone (aka da-zone). It's been awhile since I have worked on an "older" chebby. Not sure, but in 88' did they have any sort of or variation of a TPS sensor?

[Tominator]

Thanks OffRoad, Ya know I forgot Autozone renting that stuff ... I've rented from them before. Duh! I just got back from there and bought a FPG for $40.00. I figured it might come in handy down the road.

Yes it does have a TPS sensor which I haven't checked out yet. Good idea. I *** u me d that the ECM would have shown it as a trouble code.

Why do we need all of these ******* sensors anyway?!

Cant wait to build an engine that doesn't need a computer to run it!

[OffRoader]

Modules and Sensors like TPS, IAC, MAP and some 02's don't show up on OBD scans, as long as their "working". da-zone sells an ignition module/sensor tester (thay may rent it too, but doubtful) for about $60.00 keep us posted.

[Tominator]

Was just talking with a friend and thinking back to an earlier post about bad gas (no pun intended).

The gas tank has about an 1/8th of a tank of gas in it and the truck has been sitting in the garage for about a month and a half while I've been working on it and we've been seeing some fairly extreme temperature changes lately. I'm starting to think that the gas in it is the culprit especially the way it started to run rough all of a sudden after fixing the ECM.. I think the first thing to do is put a couple cans of heet in it and about five gallons of fresh gas and start there.

[1fatcat]

Don't go nuts with the "heet"! That stuff is basically 100% alchohol.

It needs to be deluted AS STATED ON THE BOTTLE!

Too much will make it run worse than it already does.

Your old 88 had a lot of troubles with the EGR valve loosing it's internal spring tention and causing the valve to open when it shouldn't. There is no good test for this. Replace the EGR valve and the EGR valve solinoid for good measure. Also check the vacuum lines to and from the valve and solinoid...replace as needed.

Your symptoms do fit the profile exactly.

[carsandcycles]

If you do use "Heat", and you have fuel injectors, use the product in the red bottle (isopropanol), the stuff in the yellow bottle is methanol and could harm your fuel system. ( I remember this tip from Sam and Dave; I think it was from "Shadetree Mechanic" or "Crank and Chrome")

[danny]

don't go crazy with additives of any kind, most fuels have a lot of chemicals already added in and adding more will only add to your problems. basically i'm saying just add fuel and hope the problem solves itself.

[Racerock]

I agree with Danny, First thing I would do is get about 5 more gallons of good fuel in there without any additives. A month and a half is nothing for time in fuel. I go way longer than that on my boat, mowers, etc with no problems. Then make sure you check the fuel pump pressure at the fuel rail. You can rig up your Compression testor to do this, or just go out and get one. Typically pumps are either good, or not - but occasionally pressure is reduced.

[KA9MOT]

Originally posted by Tominator
Thanks Beef,

That helps me narrow it down!

LOL Ya' Think! LOL

It was kinda vague wasn't it. Sorry about that. Usually I am way more descriptive.