A bit rough at idle but fine otherwise?

[Jerry]

Hello fellow CCers.

 

I'm looking for some help diagnosing a mild misfire and hoping somebody can point me in the right direction?

 

My 4.0L runs and drives great, mpg is good, pulls well but it has a persistent stumble/misfire at idle. It's not rhythmic and one cylinder specific, more like a few misses on several cylinders then smooth then a few more misses then smooth again and so on. I replaced the CPS a while back and just recently sprayed some carb cleaner down the throttle body which seemed to increase idle speed but made no difference to the misfire.

 

The truck has 118k miles on it, starts easy and runs well when driving. Although, I guess it may still be missing further up the rpm's just I can't detect it.

 

Any ideas? Injectors maybe? What else should I look at?

 

Thanks in advance :)

[Incommando]

On mine it was the CPS. It got progressively worse and eventually starting stalling. I replaced the CPS and it corrected all of the problems including the random misfire at idle just as you describe. Considering that many aftermarket CPS units seem to have a higher failure rate than the original I would not rule that out as a culprit even if it has previously been replaced. I will point out that I have a '91 HO and not a Renix so that may make a difference. 

[Jerry]

Thanks for the input sir.

I do have my suspicions about the quality of the new cps that I installed, it was very cheap but it was the only choice. Think I'll order another from a different supplier and see if that helps.

 

:)

[Incommando]

RockAuto has some higher end choices that are a lot cheaper than the better ones that I can buy locally. I do not know what duties and shipping to the Great White North may do to that savings, though.

[Jerry]

Cool, I'll check that out.

[gogmorgo]

RockAuto takes care of the duty and includes it in their shipping cost. Usually I find that after shipping they'll equal local part store prices, but the selection is better.

[Jerry]

Thanks guys, fingers crossed this fixes my misfire :)

[91Pioneer]

I've found it really depends on the part and which warehouse it comes out of that affects shipping. I priced out a rad on there today, shipping was $21, but it said it ships from Canada. I picked a different rad (didn't say where it was shipping from) and shipping for it was $61. I also buy auto parts locally, from eBay, and Amazon. Once you have the part number of what you're looking for it takes less than 5 minutes to price shop online and a few minutes more to call locally.

[cruiser54]

CPS won't cause an erratic idle on a Renix.

 

Sounds like you may have a lazy oxygen sensor switching from rich to lean slower than it should.

 

Of my Tips, which ones have you done? 

[Jerry]

CPS won't cause an erratic idle on a Renix.

 

Sounds like you may have a lazy oxygen sensor switching from rich to lean slower than it should.

 

Of my Tips, which ones have you done?

 

Both, and cleaned connectors while I was in there.

 

Is there anything else i should look at? Injectors maybe?

[cruiser54]

 

CPS won't cause an erratic idle on a Renix.

 

Sounds like you may have a lazy oxygen sensor switching from rich to lean slower than it should.

 

Of my Tips, which ones have you done?

Both, and cleaned connectors while I was in there.

 

Is there anything else i should look at? Injectors maybe?

 

Both?

 

I say you need to do 1,3,4 and 5. 

[Jerry]

Both?

 

I say you need to do 1,3,4 and 5.

 

OK, now I'm confused!

1,3,4 & 5? Whatchya' mean mr?

:)

[cruiser54]

 

Both?

 

I say you need to do 1,3,4 and 5.

OK, now I'm confused!

1,3,4 & 5? Whatchya' mean mr?

:)

 

Click the link to my Tips right below here in this post.

They're numbered.

[Jerry]

Awesome, not seen your list before, what a valuable resource! Thanks man, I'll be hitting that up for info today.

[cruiser54]

Awesome, not seen your list before, what a valuable resource! Thanks man, I'll be hitting that up for info today.

Do those suggested first. Free and easy. 

[Jerry]

Will do, thanks again :)

[cruiser54]

Will do, thanks again :)

Any questions/concerns, post them up. I'm subscribed.