m42 rough idle - need help

[Dasfrogger]

The below video is of my 91 318is m42. it's got a rumbly idle (kind of like it's got giant cams), and searches between 500 and 800 rpm at idle - sometimes drops a bit more. Never dies though. Runs and drives with no issues otherwise.

I've also noticed quite a lot of air coming out of the oil filler cap and a change in the idle before and after removing it. Does the m42 have really high crank case pressure, or do I have some blow by? You can see the air coming out of the hole as well as oil splatter. If it is just high pressure, does the change in idle indicate another problem?

Any ideas on what my problem(s) could be?

Took this video tonight.

https://www.youtube.com/watch?v=OUPWxJy7KPY&feature=youtu.be

*146,100 miles on the stock motor.

* I've done the mess under the intake recently and have eliminated any vacuum leaks. Double checked via carb cleaner method and wasn't able to affect the idle.

*Stock injectors should be very clean - run a bottle and a half of sea foam and some lucas injector cleaner.

*Cleaned AFM, Throttle body, and ICV.

* Installed spare AFM with same results

*Unplugged O2 sensor same results. Installed new sensor with same results (however the new sensor's wiring was about 4 inches short and was clearly the wrong part. It has since been uninstalled)

*the random plug that makes the car run bad is unplugged

* Oil and filter are fresh 1k miles ago.
* Spark Plugs fresh 1k miles ago.

[kwill22]

Engine doing that when you remove the cap is normal. Same as if you disconnected your ICV. It throws off the vacuum and the engine doesn't like that. Which leads me to believe that you do have a vacuum leak somewhere and the searching idle is your ICV trying to compensate. Is the plug in the boot? One hole for the ICV hose and one is plugged.

Unmetered air entering the engine will do this. I'd say double check the hoses again.

[Dasfrogger]

Engine doing that when you remove the cap is normal. Same as if you disconnected your ICV. It throws off the vacuum and the engine doesn't like that. Which leads me to believe that you do have a vacuum leak somewhere and the searching idle is your ICV trying to compensate. Is the plug in the boot? One hole for the ICV hose and one is plugged.

Unmetered air entering the engine will do this. I'd say double check the hoses again.

I figured the change in idle may be normal. Is the amount of air rushing out normal due to high crank case pressure or does that indicate blowby?

Plug is in the boot. I'll double check the hoses again too.

Thanks!

[romeomike]

Hi:

Assuming your username is the same here as on m42club.com, you've read my posts there on this topic. Prior to getting a valve job, I had that light sounding noise coming from the exhaust manifold area, but my idle was much rougher, and didn't smooth out until the rpms were over 1,000. For your sake, I hope it's not an impending valve failure.

A smoke/compression/leakdown test might be called for here.

[Dasfrogger]

Hi:

Assuming your username is the same here as on m42club.com, you've read my posts there on this topic. Prior to getting a valve job, I had that light sounding noise coming from the exhaust manifold area, but my idle was much rougher, and didn't smooth out until the rpms were over 1,000. For your sake, I hope it's not an impending valve failure.

A smoke/compression/leakdown test might be called for here.

Yep same post. I posted here for a little more exposure.

Plan is to do leakdown/compression first to see what's going on there. My buddy may have access to a smoke tester for vaccuum leaks and we'll try that too.

Then we will try swapping over his know good AFM and o2 sensor one by one to see what happens.

Planning on having a set of mustang injectors cleaned and I will install them. Will do a valve cover gasket replacement at the same time. Doing these first since I already have parts on hand.

If still no luck I'll be doing timing tensioner, plug wires, coil packs (will do coil on plug swap for wires/coil). I'm trying to put off spending $$ so I'll likely test plug wires to see if they're still good before ordering the coil on plug kit and tensioner.

Hopefully that's a solid plan, and hopefully my problem won't be worse than what's listed.

[oldmann]

Sounds like your eliminating the things that will cause an erratic idle, but here are a couple more.

O-Rings on dip stick tube, Valve cover gaskets, CK the seal on the oil fill cap, Charcoal canister hoses and valve, also the brake booster and related parts-hoses. Throttle body shafts may be leaking.

CK to see if the idle adj. screws have been messed with, throttle plates open at idle beyond specs will make the ICV hunt.

Your already gonna do the valve cover and such, but all intake mainfold gaskets are suspect. Definitely do a smoke test.

[ihaveaquestion]

How do you clean the throttle body? I'm having issues with idling myself and I'd like to try this solution

[roguetoaster]

To clean the TB simply remove it and spray the butterfly valves and the vacuum ports with brake cleaner. I would imagine that your throttle is not dirty enough to cause poor operation as I have yet to see that happen on an M42.

As you are going to remove it be sure to check that the valves appear closed/seated when at 0% throttle and that they appear to open progressively all the way to full throttle.

[Dasfrogger]

How do you clean the throttle body? I'm having issues with idling myself and I'd like to try this solution

To clean the throttle body (green arrow) you need to remove the intake boot (blue arrow), then remove the 4 nuts that hold the throttle body onto the manifold. They sell a specific throttle body cleaner at your local parts store - use it.

Other things that can and should be cleaned are also marked.

Pink arrow- Air flow Meter (AFM): remove the intake boot and release clips on airbox to remove the afm with the top of the air box. This way you can clean from both sides. They make a AFM/MAF cleaner - use it. If your air filter is dirty this is a good time to change it.

yellow arrow - Idle control valve. this is a little more difficult to get to, and less likely to be gunked up. but if you're motivated, or have the manifold off, you can clean it too.

sorry if this is overly simplified for anyone. There have been a lot of times when i benefited from overly explicit explanations so i try to do the same when posting online for other folks.

If this doesn't help you can buy some carb cleaner and spray (with the car running) that at each vacuum hose junction to check for vacuum leaks. if the idle changes after you spray, you've found a leak.