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Old 04-14-2004, 04:19 PM
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Default Diagnosing 4.0l rough idle

Vehicle: 1982 CJ-7
The engine is a 1990 Renix, pre HO retrofitted with Mopar MPI system, in-tank fuel pump. The engine runs great but has an intermittent miss at idle. Itís not consistent like a burned valve. Things I have done to check it out:
Fuel pressure and flow within specs.
Hooked timing light to each plug wire and watched flash when miss occurred. Flash didnít change.
Used mechanics stethoscope to listen at each injector. Each one clicked consistently, no difference between the injectors either.
Checked for vacuum leaks, especially at intake manifold, none.
There was 20k on the engine when removed from the XJ but the injectors have 120K on them. I tried several canned cleaners and use Chevron gasoline. Iím thinking that the injectors need to be replaced or sent off for cleaning, any other ideas?

Thanks,

GM
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Old 04-14-2004, 06:55 PM
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Default Re: Diagnosing 4.0l rough idle

Was one of the cans "Sea Foam"? I haven't used it, but many consider it a miracle in bottle/can for symptoms like yours. [img]images/graemlins/grin.gif[/img]
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Old 04-14-2004, 11:26 PM
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Default Re: Diagnosing 4.0l rough idle

How did you check for vacuum leaks - procedure? Many popular "home methods" don't work.

How do the plugs look?

Intermittant - comes and goes while idling, or sometimes it does it, sometimes not?
Hot? Cold?

How 'bout just slightly up from idle - still rough? At about 1500? 2000?

Need to know more info to help.

And - always start with the cheap and easy things, your list started with the major things.

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Old 04-15-2004, 05:02 AM
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Default Re: Diagnosing 4.0l rough idle

I'd also say what do the plugs look like. If it were an injector, the miss would be regularly. So a bad injector is not likely, or they should all be bad.

You might have a slow O2 sensor. Have it checked. But even if is't working, it might be working slow.

Vac leaks cause rough intermittent idling. check all connections on the intake manifold by spraying WD40 and listen if the idling becomes regular. I once had a missing O-ring in a new set of vac lines. Check if you blocked of the non used connections at the intake manifold.

Did you ever clean the throttle body ? Espacially the bore in which the air stepper motor is can be clogged up badly. The stepper motor itself may be faulty, but mostly it causes a stalling engine instead of rough idling.
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Old 04-15-2004, 01:04 PM
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Default Re: Diagnosing 4.0l rough idle

Sorry, gotta disagree on the WD-40. WD-40's kerosene and perfume. (Good to spray on your arthritus joints - using kerosene for that is a very old home remedy.)

It doesn't burn, so it acts the same as if you sprayed water on it. At best it may plug the leak if the hole's small enough. Or it may even wash dirt into the hole enough to plug it. But it won't last.

Use carb cleaner that actually burns - most don't - starting fluid -- keep a fire extinguisher handy - don't use brake cleaner - when it burns it makes a very poisonous gas.

Best way - propane. Yes, it's flammable too - so be careful. But if it does ignite it flashes, then goes out - no puddles to keep on burning - scares hell out of you - leaves your hair on the hood latch, but goes out. I can prove that - just look at the top of my head! Looks like a road map.

Use a propane torch, remove the tip - slip a long vacuum hose over the end and probe around suspected areas. (Unlit of course.) If you aren't getting enough propane, drill or remove the orfice in the base.
It burns just as good as gasoline, and is not harmful to engines. The advantage is you are in control as to how much and where.

Works very well for tiny or large vacuum leaks - even in places you can't see - like under the manifold etc, even gets in tiny cracks too. Useful in setting mixture - and even determining "fuel vs. ignition" problems. When you hit the leak, the RPM will change.

If the vehicle is computer controlled the idle will jump up, then the computer almost immediatly drops it down again - so listen carefully. When you take the propane away, idle drops, then the computer again adjusts the speed. Ya gotta listen very carefully. If you disable the computer it goes full rich, so the propane may not react as well.

No computer? Adjust the mixture screws to very lean - to almost a lean stumble - makes it even more sensitive.

It can be used at any speed too. At higher speeds (even driving) it can help you decide if the jets are too rich or too lean too.
Do a search for "propane trick" where I posted more info on using it. Wonderful stuff!

He He - funny - Selling automotive test equipment (scopes, exhaust analyzers, dynos etc) I did lots of training - part of it was finding vacuum leaks - a very common problem. Often the "mechanic?" would tell me how "he does it" - usually not very effective. Would you believe - Some even used spray paint!
He He - One guy told me he sprayed water around - then looked for the bubbles from "the vacuum leaking out." I don't think he ever did understand what vacuum is.
He told me "almost all exhaust manifolds leak vacuum, because they bubble." He replaces lots of manifolds because of that. (I wonder if he ever retested them after he replaced them?)

One 70's Ford had over 280 feet of vauum hose on it - a nightmare!!!!!



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Old 04-15-2004, 07:02 PM
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Default Re: Diagnosing 4.0l rough idle

</font><blockquote><font class="small">In reply to:</font><hr />
How did you check for vacuum leaks - procedure? Many popular "home methods" don't work.

[/ QUOTE ]
Actually I did use the propane method.
</font><blockquote><font class="small">In reply to:</font><hr />
How do the plugs look?

[/ QUOTE ]
White, no blistering or spots of metal, yes I do have some detonation when running regular fuel.
</font><blockquote><font class="small">In reply to:</font><hr />
Intermittant - comes and goes while idling, or sometimes it does it, sometimes not?
Hot? Cold?

[/ QUOTE ]
When warmed up but haven't checked when cold. I guess you could say it comes and goes. It acts like an intermittant bad wire or plug is the only way I can explain it. Thats why I used the timing light to check if the miss related to a erattic light flash. It does it all the time. It doesn't smooth out for a while then start missing again.
</font><blockquote><font class="small">In reply to:</font><hr />
How 'bout just slightly up from idle - still rough? At about 1500? 2000?

[/ QUOTE ]
No, smoothes right out.

Thanks for all the tips.

GM
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Old 04-16-2004, 06:39 AM
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Default Re: Diagnosing 4.0l rough idle

If you have a '90 system it should be equipped with an EGR. Check all vaclines, connections, solenoid, diafram and EGR (closing) itself. You might have a cracked line, missing O-ring, dry rubber hose, name it. I reraed your post and still think it's a vac related problem. In my XJ the diafram next to the EGR had no membrane in it anymore.

Sometimes the grommet in the TB for the line to the MAP sensor can be a loose fit.

You're not having any starting problems? If so the CPS might be going bad, but from your description I don't think the CPS causes your problem.
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Old 04-16-2004, 10:51 AM
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Default Re: Diagnosing 4.0l rough idle

As I understand it then, it feels like the miss is coming from just one cylinder, -- not all of them at once, mainly hot, at idle only, goes away above idle, starts fine. And it tends to come and go?
You used a timing light to check plug wires, and you didn't find a vacuum leak with propane.

Q-- When you put a small amount of propane in the throat, did it "really like it" or not have much effect. If it "Really liked it" indicates there's still a vacuum leak somewhere.

If the EGR was leaking, slightly open from carbon, it would act like a vacuum leak the way it feels, but propane doesn't help. The exhaust gases recycled by the EGR valve don't contain enough oxygen for the propane to combine with, so you don't get much, if any reaction like a genuine vacuum leak. An EGR problem feels, looks, acts like a vacuum leak, but isn't! It's "dead" air.

The timing light method - the light will flash even if the plug wire's pretty bad. Completely open it won't, and it won't tell you if the plug is actually firing or firing properly.

I'd toss in another set of plugs (sometimes a plug cracks on the side, spark jumps the wrong way and doesn't ignite the fire properly.) Make sure you use the proper plugs for that engine - check the service manual. So often the so-called "trick" plugs cause problems, like plats, splits, multi's, etc, or the "one size fits all" types like Accel, Bosch, reds, blues etc. They are made to sell, not to use. The engineers designed it around a specific plug, use it. -- If the problem's still there you now have a spare set.

Then a set of wires - unfortunately there really isn't a good way to test them without a scope. Testing by Ohmmeter might indicate one's bad, but often will fool you. A break in the wire still has a carbon track across the gap - the ohmmeter "sees" the carbon track and reads fine, yet it cannot carry the amp or so of the ignition spark.

Cheap way - get one plug and one wire -- swap them around - lots of work though. Some day you'll need full sets anyway.

If it feels like it's just one cylinder (like thump thump thump) look for something that can only affect one cylinder (not too many things that can do that.) If it seems like it's several cylinders, look for something common to all cylinders. Cap rotor, coil wire, coil etc. I rather doubt it's caused by the coil or coil wire though, as it runs fine at higher speeds and under severe load.

You can isolate it to one cylinder by pulling one plug wire at a time and listening for the least amount of change. A good one will drop the R's. Listen close, often a computer system will compensate for the RPM drop, so the change will only be momentary.

Sometimes you'll find when you pull one cylinder's wire, RPM's go UP! Sounds goofy, but that's often an EGR problem, or ignition crossfire.

Not much, if any chage indicates you've got the right one.

So Mr. Sherlock Holmes, get out that magnifying glass and go searching - let us know what you find.

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