Rough Idle & Off-Idle Hesitation

[stefano973]

Hello everybody.
Darrel, have you solved the issue?

I have exactly the same problem with a 1987 320i Cabrio with L-Jetronic injection (no ICV, no catalytic converter, no O2 sensor, mechanical ignition with centrifugal and vacuum operated timing on the distributor…), stable idle at 850 rpm but my engine still runs rough at idle and shakes following a non regular pattern, even after a complete overhaul of the engine (crankshaft bearings, piston rings, valve guides, valve seals, valve seats, head gasket, engine mounts, etc…) and replacement of many parts :

new spark plugs
new spark plug wires
new distributor cap and rotor
new coil
new fuel pump
new fuel filter
new fuel injectors (refurbished, tested and flow matched)
new air filter
new timing belt and tensioner

The roughness seems to be due to a not regular combustion that is very noticeable at idle and gets smoother at higher revs, disappearing after 1600-2000 rpm.
Actually it is not a complete misfire, like disconnecting a single injector or a single spark plug. In that case the roughness and shaking is very strong and regular. It is more a slight combustion defect, with a random intensity and timing.
It is weaker when engine is cold or warming, seems to be a bit worse when engine is completely warm and stopping at idle after a long trip.

For sure it is not a fuel delivery issue related to fuel pressure regulator, pump or whatelse because the fuel pressure is OK at 2.5 bar and if I remove the fuel pump fuse while the engine is running, to stop any pressure variations due to pump and fuel pressure regulator working, it keeps shaking at exactly the same “rhythm” for some seconds, before it dies as expected.

It is not an air/fuel ratio issue: leaning or enriching the mix through the idle CO screw or forcing the position of the AFM potentiometer doesn’t solve the problem, clearly the engine runs worse exaggerating the correction in both directions.

It doesn’t seem to be due to a vacuum leak, the vacuum is high and stable in the intake ( -670 mbar at idle ) , a slight enrichment of the mix doesn’t smoothen the idle.

Definitively it is not an electric / electronic related issue, after changing most parts I measured everything with an oscilloscope: primary and secondary ignition waveforms, fuel injector voltage waveform and current ramps, air flow meter control voltage, inductive trigger pickup on distributor. All waveforms show regular patterns and values as expected.

Ignition timing is OK and stable, I checked it both with a timing light gun and then with an oscilloscope with a TDC reference and a capacitive probe on ignition wire of cylinder 1.

I tested with a pressure transducer the pressure in both the intake manifold and in the exhaust as well.
The intake pressure waveform associated with openings/closings of intake valves is very regular and even.
The exhaust pressure waveform on the contrary is relly uneven, the pressure waveform varies continuously from cycle to cycle and from cylinder to cylinder, the same cylinders show a quite regular pattern on a cycle and a irregular pattern the next cycle, confirming what seems to me an irregular combustion problem.

The exhaust gas analyzer doesn’t show any problem: CO 2,9%, lambda 0,92, CO2 13%, HC 250 ppm .

Crancking compression is OK, 11 bar , the same on all cylinders.

Valve clearance was adjusted two times: the first time before engine overhaul, then some time after engine overhaul because the engine was really rough.
Strangely, both times the mechanic told me that he found very tight valves, without any clearance. Last adjustment improved things a bit, but didn’t solve the issue.

Finally I ordered a high pressure transducer to measure running in-cylinder compression and detect any valve / cylinder leakings or cam timing issues with the scope.

Help me, I spent a lot of time and money and I’m still at the starting point!!! This car is bewitched….

Stefano

[Rontgen]

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

Out of interest....what's the state of the C191 plug connections and wiring ?

The round plug under the intake manifold. That plug feeds spark to the injectors, corroded wiring in C191 isn't unusual and can result in poor running if rot is causing high injector circuit resistance.

Might be the cause of your slightly lean condition.

Sorry, I totally missed these replies somehow. I haven't had a ton of time to work on this and my original smoke machine order from eBay didn't go through as planned (long story). I have another smoke machine on order from a different seller and should receive this one soon. Once I get it, I'll check for vac leaks, although I suspect I won't find anything. Famous Last words, right? :)

I cleaned the C191 today. Not a lot of corrosion, but there was a little bit. First start from cold seemed smoother than before, but that may be placebo effect. We'll see how it does tomorrow after sitting overnight.

Thanks for your suggestions!

[Seawolf]

Out of interest....what's the state of the C191 plug connections and wiring ?

The round plug under the intake manifold. That plug feeds spark to the injectors, corroded wiring in C191 isn't unusual and can result in poor running if rot is causing high injector circuit resistance.

Might be the cause of your slightly lean condition.

[Z06&E30]

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

[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!

[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]

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!

[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.

[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?

[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!

[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]

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]

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

You aren't using old fuel by any chance?

[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.

[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!