Rough Idle & Off-Idle Hesitation

[Rontgen]

I've been trying to chase down a problem with my 90 325is which has a freshly-rebuilt M20 and could use some help from some of the R3v M20 whisperers. :)

I bought the car about a year ago and the M20 ran, but ran very poorly. Compression check showed two questionable cylinders and it burned quite a bit of oil via the valve seals. Idle was rough and it just generally ran like shit until you got above about 1500rpm, which I attributed to the blow-by. I pulled the engine 6 days after I bought it and did a complete overhaul with new rings, bearings, etc. I currently have about 500 miles on the refreshed engine.

Overall, it runs pretty good - no more burning oil, no more leaks, but the idle is still very rough and it still tends to stumble just off idle (800-1500rpm), particularly when cold. Once the engine is fully warmed up, the off-idle hesitation improves quite a bit and at all temperatures, it runs great above 1500rpm. It pulls all the way to redline and while I wouldn't call it "fast", it seems about right for 160 crank HP.

THE PROBLEM: When the car is idling roughly, you can feel and see the engine shake back and forth. You can also hear a "burbling" or "choppiness" in the exhaust note. When you rev the car in neutral, you feel a shake that speeds up with rpm and then smooths out around 1500rpm or so.

However, while the idle is rough and shakes the car, the rpm NEVER fluctuates. It's rock solid at any temperature and any driving condition. It never dies, never surges, never dips - nothing. When I turn on the A/C, the idle rises slightly like it should and is just as steady. Aside from the shaking, I'd say it idles perfectly. The car starts every time and never dies after starting, even when cold. It's only when you try to blip the throttle (especially on a cold start) that the engine falls on it's face.

I've done the following so far:
- Compression check: 157-160psi on all 6 cylinders (dry)
- Verified Timing belt is installed correctly
- Verified spark to all 6 cylinders by using timing light inductive pickup
- At idle, it looks like the engine is running around 20 degrees timing advance referenced to cyl #1 (which sounds reasonable based on previous experience with American V8s)
- Inspected harmonic balancer tooth ring - no signs that it's spun or moved relative to the hub.
- Replaced/Inspected all spark plugs - all look a little on the lean side, but they all look the same
- New cap & Rotor (Bremi)
- AFM maintenance (clean contacts, adjust for new track, verified with Fluke Scope Meter, adjust idle mixture screw to factory stamped depth)
- 3-wire IAC valve is clean and coil resistance matches specs in Bentley.
- Checked for vac leaks more times than I can count. Sealed up every vac line, no matter how unlikely a leak appears.
- TPS switch is new (Bosch), and adjusted properly
- Throttle cable is adjusted so no excessive slack
- Valves have been adjusted several times and are not excessively noisy.
- New Bosch O2 sensor
- Fuel injectors were sent to MEPEH for cleaning and rebuild with new O-rings & pintle caps
- New Fuel Pressure Regulator (supposedly 3.0 bar, but I need to verify)
- EGTs are all 235-250C as measured with an infrared heat gun.
- Catalytic converter is new Magnaflow
- Engine Temp Sensor measures normal when cold, both at the sensor and at the DME.
- New Fuel filter, fuel lines, etc.

Ideas? Suggestions?

THANK YOU!

[Mook613]

Could be something in the head.
Check the cam for wear spots.
Also could be the rocker, eccentrics, or too much wear on the top of the valve tips (which makes the standard gapping on an adjustment a little off).
Check your engine mounts as well.

[dnguyen1963]

You have done pretty much everything that I can think of. Check the DME to make sure that the chip matches up with the fuel injectors. If you have the wrong chip, it can mess up your idle. Good luck.

[Rontgen]

Thank you!

The head was replaced during the engine rebuild with a rebuilt 885 unit that supposedly meets all specs and so on. Valves are currently adjusted to .007" between the cam lobe and the rocker (which yields around .011" at the valve tip) in order to eliminate worn eccentrics/valve tip as a suspect.

I forgot to mention above, the car pulls about 16" of vacuum at idle with no fluctuation. (I'm about 1000' above sea level, so it should pull 17" at sea level, which is right on the edge of generic 4-stroke normal)

Engine mounts are new, as are the transmission mounts ( both OEM Replacements).

I'll take a look at the DME, but the car was purchased bone stock from an older guy, who inherited it from an even older guy who passed away. I'll definitely verify it, but I would be shocked if either the DME or injectors had been changed at any point.

Thanks again - I really appreciate the input!

[Rontgen]

Had just a little bit of time this morning before Mother's Day brunch, but managed to cross a few things off the list:

- Valve adjustment looks spot on. 0.010" feeler gauge fits between the valve tip and rocker with a slight drag. 0.012" will not fit.
- Cam lobes all pass visual inspection
- While running, if I pull one spark plug wire at a time, the idle becomes even rougher. Leads me to believe that the miss isn't ignition related, although I may invest in spark plug wires and a coil for preventive maintenance.
- Spark plug gap verified at 0.032"
- Pulling the FPR vac line (and plugging the resulting vac leak with my finger) should instantaneously richen the mixture considerably, yet appears to have little/no effect on the rough idle.
- Pulling the dipstick creates a vac leak, which makes the idle dip and roughen even worse.


Next up, smoke test, fuel pressure tests, and possibly some new ignition components.

[e30davie]

Damn you have pretty much replaced everything, bloody thing should be brand new.

You aren't using old fuel by any chance?

[Rontgen]

Tell me about it! This thing should run great, but it just doesn't. Although I didn't drive the car much before I pulled the engine, I believe this problem was present before. I attributed it to the oil consumption/blow-by and the various oil/crankcase vac leaks that the car had. Obviously that wasn't the whole story. Therefore, the problem should be something that hasn't been replaced (plug wires, coil, CPS, AFM, etc). I feel confident I'll eventually get it - it's just a question of how much time and $ I'll blow in the meantime. [emoji23]

I've run through 3-4 complete tanks, so any old fuel should be long gone by now.

Thanks - appreciate the input!!

[e30davie]

If pulling the dipstick makes it worse (more vac leak), although its the generic answer, but i reckon you might still have a sneaky vac leak somewhere.

Is there anything on the intake manifold that you have not taken off and resealed?

The little plate above the tappet cover on the intake manifold was leaking on my car, the bolts that hold it on were tight but not tight enough, 1/2 turn on each bolt was enough to tighten it up and stop my roughish idle.

How about the gasket on the throttle body? You change/check that?

your hesitation off idle might be related, a lean condition causing a misfire when you put your foot down. Although it usually takes a fairly bad vac leak to have on throttle issues:S

[Rontgen]

I've ordered a smoke machine from eBay - should have it by the weekend and we'll see once and for all where this thing leaks!

Throttle Body gasket is new, and that end plate (where the FPR vac line connects) has been resealed with a skim coat of RTV. I think all of the intake connections are solid, but I guess the smoke machine may make a liar out of me! :)

I agree - the car acts like it's lean at throttle tip-in as it falls on it's face and recovers. Could very well be related to the idle misfires. Perhaps a fuel delivery issue, perhaps a result of a vacuum leak. We shall see!

Thanks again!

[ahlem]

What's your fuel pressure? Did you change the fuel filter? How old are the rubber hoses in your fuel system? Did you take a look at the intake screen on the end of your fuel pickup in the tank?

[IceWhite]

I had a problem with off idle hesitation too. It turned out to be the throttle body metal fittings. The one which the vacuum hose from the brake booster attaches to the throttle body.

[Rontgen]

What's your fuel pressure? Did you change the fuel filter? How old are the rubber hoses in your fuel system? Did you take a look at the intake screen on the end of your fuel pickup in the tank?

Fuel pressure is still on my list of things to check. I have a gauge, but don't have a 5/16" inline adapter. Once I track down one of those, I'll verify.

I replaced virtually every hose I could get to. New feed lines, new filter, new lines near the filter, and quite a few of the EVAP lines. The only ones I did not replace were the ones that connect to the top side of the fuel pump. I haven't pulled the fuel pump out yet, but maybe that would be a good idea at this point. There's a possibility that it's the original fuel pump, and if that is the case, I wonder if either the sock is clogged or that rubber hose is rotted. Hmm

Thanks for the suggestions!

[Rontgen]

I had a problem with off idle hesitation too. It turned out to be the throttle body metal fittings. The one which the vacuum hose from the brake booster attaches to the throttle body.

Thank you! Those appear tight on mine, but hopefully the smoke check should confirm that.

Thank you!

[Rontgen]

Small update:

Got the correct fitting so I could check fuel pressure. First, I pulled the relay and jumped pin 30 to pin 87 to run the pump with the engine off. Fuel pressure rose very quickly to 44psi (3 bar) and held steady.

I put the fuel pump relay back in and started the car, fuel pressure at idle is 37psi, and rises to 44psi at wide open throttle or when the vac line is disconnected. No fluctuations at all.

Once the engine is turned off, the pressure holds for quite a while. With a starting pressure of 36psi, it was still at 34 when I checked it 30 mins later. So, no regulator leaks, no leaky injectors, and no pump check valve issue.

On to ignition and vac leaks!

[Z06&E30]

Any progress made on this??? Chasing something similar.