Chasing a rough idle

[Burbman]

'98 L36. Just finished the LIM gasket, which included a new UIM and gasket, new PCV, new water pump, plugs, wires, thermostat, air filter, throttle body cleaning and fuel injector servicing/cleaning.

Had a very rough idle before the project, now a little better but still rough. Never sounds likes like it's gonna stall, but you can feel the vibration in the seat of your pants more than the steering wheel. Shifting to neutral makes no discernable difference.

Scanned it and no codes, no hesitation on acceleration, no drivability issues. Only thing I could find was the purge valve started a fast ticking noise when the idle started to go rough, so I'm thinking of dropping $25 on that.

Idles smooth when first started, doesn't get rough until she's been run a while and waiting at a light.

Any ideas, or is this is as good as it's gonna get. 153k miles, no mods.

[FordMan77]

Maybe check for vacuum leaks. The EGR tube from the LIM also likes to split and hides pretty well since it's often in between the accordian sections.

[Sushamouf]

You could try unplugging the maf or o2 sensor while its idling to see if that changes anything.

Sent from my Droid

[Fivefingerdeathpunch]

What brand plugs and gap?

[nos4blood70]

Clean your throttle body if you haven't already. It did wonders for me.

[GTP in RTP]

Maybe check for vacuum leaks. The EGR tube from the LIM also likes to split and hides pretty well since it's often in between the accordian sections.

Thats what mine had, I also replaced the screen in the tb. Helped a lot.

[Burbman]

Thanks guys, will check and report back.

[GTP in RTP]

'98 L36. Just finished the LIM gasket, which included a new UIM and gasket, new PCV, new water pump, plugs, wires, thermostat, air filter, throttle body cleaning and fuel injector servicing/cleaning.

Had a very rough idle before the project, now a little better but still rough. Never sounds likes like it's gonna stall, but you can feel the vibration in the seat of your pants more than the steering wheel. Shifting to neutral makes no discernable difference.



Scanned it and no codes, no hesitation on acceleration, no drivability issues. Only thing I could find was the purge valve started a fast ticking noise when the idle started to go rough, so I'm thinking of dropping $25 on that.

Idles smooth when first started, doesn't get rough until she's been run a while and waiting at a light.

Any ideas, or is this is as good as it's gonna get. 153k miles, no mods.

You have a ticking purge valve ?

[Burbman]

Yes, purge valve is ticking. What's that EGR tube thingy called? I don't see it on the Rock Auto catalog.

[GTP in RTP]

Yes, purge valve is ticking. What's that EGR tube thingy called? I don't see it on the Rock Auto catalog.

EGR tube, looks something like this. Don't think RockAuto sells it. JY ?

[Burbman]

Found the EGR tube on flea bay, guy selling the OEM part for $25 shipped. My understanding of the purge valve is that it's only supposed to open at highway speed when it dump accumulated vapors from the charcoal canniseter into the intake where they are diluted and burnt. Mine goes tic-tic-tic at idle and if you put your hand on it you can feel the solenoid doing the jitterbug. Rock Auto $16 so its gone. Unplugged the MAF while idling. Car immediately ran like @$%, threw a code and lit the SES light. Reconnected and 4 engine cycles later the SES light is out.

Plugs and wires are new, I changed them trying to get rid of this rough idle before I did the LIM job. Plugs are AC Delco Iridium gapped at .060 IIRC.

I cleaned the TB when I had it off, but wasn't able to get it very clean, it was really caked on. I need to remove to get to the EGR tube anyway so I will take another pass at that.

[BillBoost37]

Purge valve can open at idle. There's criteria to be met and it'll open. What it's doing when it does open is allowing more unmetered air into the engine.

How does the fitting at the TB look where the evap and FPR come together?
On the EGR tube, it's common, super common. It cracks right at the edge where it's held to the LIM. Slide the piece that is bolted to the LIM back and look at the edge of the tube where it sits. See any black line or jagged line, now spin it around and look into the tube, see light....sure you do.

[Burbman]

I would think that if the purge valve is open or closed, it shouldn't make a tic-tic-tic sound. Sounds like the solenoid inside is chattering...maybe that's normal but doesn't seem like it should be. I pulled the vacuum tube coming off the purge valve, and there is strong vacuum at the hose going to the purge valve, but it didn't make any difference in the idle with the hose on or off.




I'm not even looking at the EGR tube, I'll take your word for it. You were spot-on about the LIM and UIM, so no reason to question this, the part is ordered and I'm just going to change it. I'll report back and post a pic after I get the old one off.

How does the fitting at the TB look where the evap and FPR come together?

Gimme a little direction here, not sure exactly what fitting you're talking about.

[BillBoost37]

When you pulled that end off, the IAC compensated for your action with a reaction.

Follow that evap tube to the TB. That fitting. Should be a goofy rubber piece with the tube for the fpr and evap tube both stuffed into it.

On the evap ticking. I'm pretty sure that is the normal operation..call it the way an open or closed valve can meter the amount of vacuum put on the tank. I googled a few evap codes and it does indicate that clicking should be heard when it's operating.

Then found this. It's pwm controll witll click

The EVAP purge solenoid controls the flow of vapors from the
EVAP system to the intake manifold. This normally closed
solenoid is pulse width modulated (PWM) by the control module
to precisely control the flow of fuel vapor to the engine. The
solenoid will also be opened during some portions of the EVAP
testing, allowing engine vacuum to enter the EVAP system.

[Sushamouf]

X2 on the evap clicking. It did for me on the GTP and previous vehicles.

Sent from my Droid

[Burbman]

Anybody wanna buy one? LOL, so much for intuitive diagnostics....

[BillBoost37]

I feel this is like someone choosing to buy an O2 because it's going up and down rapidly on the scan tool. Really gotta find out what normal is before saying it's broken.

All women would be broken if we went with our intuition about what normal should be ...lol

[Burbman]

I think "normal" is an obsolete word...I keep forgetting that.....

Now that you mention it, I thought all women are broken....hmmmmm, sweetie would probably ask me to rethink that LOL

[Burbman]

Progress has been made. I replaced the purge valve and the EGR tube. I was able to get the EGR tube off without removing the throttle body, but realized there as no way to get the new one on, so off came the TB. Interesting side note, a small amount of coolant leaked out when I removed the TB, and I remembered that those passages were dry when I removed the TB prior to the LIM job. Tells me the LIM is now tight, as it's holding coolant, whereas before it was just leaking out.

Anyway, no rust at all on the old EGR tube. Checked for breaks, none to be found, seemed to hold pressure when blowing in one end with my finger on the other end. However, the ends that connect to the EGR and the LIM have crush-type seals, and they were plenty crushed, so even if the old EGR tube wasn't broken per se, there is a good chance it wasn't dealing to the LIM adequately.

Replaced the purge valve, and guess what: no ticking sound. So if yours "ticks all the time" don't assume it's working properly. Also found that the elbow vacuum fitting coming off the small tube from the purge valve was cracked, so replaced that.

Finally, since the TB was off, I removed the IAC (had not done previously) and cleaned it up good with MAF cleaner. Lots of carbon deposits on both the solenoid end and the TB itself where the solenoid seats.

Sooo....the final result is a decent improvement in the idle quality, maybe 65% improvement. Not smooth as glass, but significantly better than it was. Not sure I'll be able to dial it in any better with 153K on the clock. I'm guessing there wasn't any one thing per se, but each "fix" provided some incremental benefit. Also seems to have a little more zip to it power-wise....

[GTPRIX99]

Did you check the ICM that was the problem for my rough idle.