Automotive → 1998 Chevy S10 4.3L High Idle Problems - Need Help!

Can you help me figure out my 1998 4.3 liter S10 idle problem? So when I start it up it idles from 1500 to 2000 rpms in park or neutral. When I put it in drive or reverse the idle goes below 1000 and the engine sounds like it is going to die it is missing so bad. When I drive it down the road the engine runs great but at an extremely high idle that it almost seems to have a somewhat cruise control mind of its own. I have checked all vacuum lines, IAC, MAF. I was getting a code of #4 misfire so i changed out the spider injector and coil inside the plentum for #4 and it still does the same thing. I am thinking there might be a crack in the intake gasket even though there is not any antifreeze puking out or in the engine? Also I tested the IAC by removing it from the throtle body and keeping it plugged in then turning the key on and seeing that the valve expands out and when i turn the key off it contracts in. I think this is the appropriate method of testing. Also there is good tested compression in all cylinders.

Have you pulled out the spark plug for cylinder 4? At first I would suspect a vacuum leak too, but normally a vacuum leak won't cause a misfire in only one cylinder.

I would try to look into the ignition system for loose plug wires, broken or burned connectors. Then check your distributor cap inside and see if you have any excessive wear on the rotor button or evidence of burning inside the cap (carbon black traces). Look at the plug and see if it looks carboned up.

Misfires are more common under low RPM conditions where the vehicle is under load. They don't do it as much at idle in neutral or when driving. If that cylinder is misfiring bad enough, it could be pouring unburned gas into the exhaust which can mess up the air/fuel mixture and cause the ECM to compensate by leaning the mixture down too much which can cause the whole engine to run rough. Does it misfire more when the engine is hot or cold?

The best way to diagnose it would probably with an engine analyzer where you can look at the spark waveform on an oscilloscope. It would show you very quick where any misfire may be and whether it is the wires or the ignition module itself.

If you see anything that looks excessively worn or burned as in the plug, wires, cap or rotor, I would try replacing them. If that still doesn't fix it then the problem could be in the ignition module, distributor hall effect pickup or in the ECM itself. Best to start with the simple/cheaper things though.

[edit] Nevermind on the distributor and wires... Didn't realize this was a DIS system.

TWO words....

Vaccum leak.

With similar symptoms on my wife's 96 S10, our culprit turned out to be a carboned-up throttle body. Of course, We didn't use ole school thoughts initially but instead leaned on the codes too much in our troubleshooting. Learned a lot about OBDII during that time period tho

Has the intake manifold gaskets ever been replaced? They are famous for blowing out on these engines causing vacuum leaks and causing coolant to leak into the engine. Been there and done that on our 98 K2500 with the 5.7 350.

So I changed the Intake Gasket and it fixed my coolant leak on the outside of the engine but not the idle/miss problem. I figured out the throttle body idle screw was tightened prohibitting the throttle to close fully. I backed off the screw and the park/neutral idle became normal 800-1000, although I still have a miss in the engine only when the truck is underload, drive/reverse. I noticed where the idle set screw was tightened in there was a small crack in the throttle body holding the screw in. The housing around this screw actually broke so I am going to be purchasing a throttle body. Like I said my idle in park/neutral is fine with a very very slight miss but with the engine underload, drive/reverse there is a more noticeable miss. The check engine light came on and I found out there is a miss in cylinder #4 code PO304 I believe. There are still all new tune up parts as well as a new line/injector for the spider injector system in cylinder 4. I think my piston rings in that cylinder might be stuck. I will have to do a compression test again in that cylinder and compare them to other cylinders. Any other suggestions?

If there is a miss only under load, it is most likely secondary ignition.

Sorry I don't know where or what you mean by that. Can you elaborate?

Secondary ignition is plugs, plug wires, distributor cap and rotor when so equipped, and coil wire. Anything before that such as coil, ignition modules, pickups, points, (remember those?) position sensors and such are considered primary ignition.

Did you replace the throttle body gasket when you did the intake? The potential of a vacuum leak at the throttle body causing a single-cylinder misfire is remote. If you are noticing the misfire at idle with no load, then low cylinder compression is a potential cause, along with others noted, although you already stated that it was OK?

I also find it unusual that you needed to change the idle screw position, as that setting is only used to locate the throttle plate, and the IAC takes care of the speed control. That tells me there is still a vacuum leak somewhere.

You are right about but I am waiting to get a used throttle body because when someone screwed in the idle set screw they broke the housing. The throttle valve which opens up when you push the gas down isnt completely closing. I think this is why the truck underload is missing. And for some reason it shows cylinder 4 just because. Otherwise I will have to check my compression again once I change the throttle body and the problem still persists. The whole primary ignition system is all new and I know there isnt a problem with any of that. I have changed plug 4 to 2 and vice versa as well as wires and there is not problem. Like I said before there is a new cap and rotor.

Sorry I ment the secondary system.

Is it possible that it's actually a floating skip (random cylinders) that the computer is pegging on #4? Pulling the plug wire on #4 while running would tell you that.

Back in the day b4 Chinese made telephone I would always assume a new phone was good but now adays new out of the box doesnt mean squat too me. I still suspect it.

So, I would want to know its its a hard or floating skip....The floating skip could be fuel related, a hard skip would make me start pulling schitt apart.

I will pull the plug wire but I had alternated wires by using wire 2 and using it for 4 and vice versa. I will check it again while running. I did a fuel pressure test and the fuel pressure is fine.

The position of the throttle plate can NOT cause a misfire. The position of the throttle plate controls airflow through the throttle body to *all* cylinders, and its position at closed throttle helps determine base idle speed, and it is a specific setting not subject to adjustment under normal conditions.

And, it doesn't cause the misfire on #4 "just because". The misfire detection routine in the on-board computer system is VERY accurate and can be trusted in nearly all situations. If it were a random misfire, then there is a code for just that--- P0300. A misfire code isolated to a single cylinder is very easy for a professional to diagnose, and you may need to go that route before you go broke replacing parts on uneducated guesses, and other bad advice.

Thanks but it is specific. It is code P0304 misfire cylinder #4.

I know that. Did you read my post?

That means it is related to that cylinder only. Ignition problem, fuel injector problem, mechanical problem isolated to Cylinder 4 only. It isn't the throttle plate.

Yes I read your post I was just saying again that it is a specific cylinder problem. I have to change the throttle because the housing is broke on it and the plate isnt fully closing even if i remove the idle screw. It is like someone bent the idle plate. This will fix my slight high idle.

The cylinder miss is probably either a bad valve or stuck piston ring. I will diagnose this after I change my throttle body. It isn't fuel related because the pressure is accurate and there is a new injector/straw for cylinder 4. When I manually turn the throttle with my hand, forcing it backwards the idle comes to its normal idle. I can see old marks where the throttle plate should be resting and it is under it whereas there is a little bit of air getting sucked into the manifold giving it a little higher idle in park or neutral.

I understand the cylinder misfire is NOT related to the throttle body.