M20 Help Needed

[Mitchen]

Okay so this spring/summer has bought about a new set of gremlins and I’d really appreciate some help with it.

Car is a facelift M20B20.

For the last few years I’ve had an ongoing issue where the engine cranks for a while before starting, sometimes it’s worse than others.. and giving it a bit of throttle usually helps it turn over. It was an annoyance in the past but didn’t hinder me enough to go fault finding. This year it’s got worse. It happens when hot and cold and the engine is really lumpy whilst I pull off the driveway.

If I give the throttle butterfly a tug, when the revs come down its struggles a bit before idling correctly and then running.

So things I’ve replaced or done whilst this fault has been present. Fitted a new blue temp sensor in the winter when i replaced the whole thermostat housing.
Fitted a new ECU earlier this year for various reasons.
Recently changed all the water hoses and did some air hoses whilst I was at it.
I carried out a smoke test the other day, nothing to report in terms of air leaks.
I suspected it could be the ICV so I got a new one from BMW this morning. Issue is still present.

I don’t know where to start with anything else other than the AFM.. I guess possibly the injection system, but the injectors were refurbed 10-15k miles ago (2016), pressure regulator replaced around the same time.

Any ideas anyone?

[flybynite]

I take it you have done the plugs and tested the plug leads? Checked the rotor arm and dizzy cap for cracks and tracking?

I am still leaning towards vacuum leak as they can be hard to find as only a small leak can cause problems and difficult to see on a smoke test. Try brake cleaner or remove some different vacuum pipes and listen for a change. No change = leak

You running it on E10? Have you checked all the fuel pipes?

[BenHar]

The injectors may be leaking so it loses pressure when stood. That would explain the long cranking time.

Ben

[Mitchen]

I take it you have done the plugs and tested the plug leads? Checked the rotor arm and dizzy cap for cracks and tracking?

I am still leaning towards vacuum leak as they can be hard to find as only a small leak can cause problems and difficult to see on a smoke test. Try brake cleaner or remove some different vacuum pipes and listen for a change. No change = leak

You running it on E10? Have you checked all the fuel pipes?

I took the plugs out and they look fine. To be honest again the cap, rotor and HT leads are all about 20k miles old. Be strange for them to fail already.

In terms of E10, I do use it, and I did just replace the two 8x13 fuel hoses to the regulator and fuel rail. I’m leaning more towards the injectors tbh. Would explain why opening the throttle when cranking helps it start.

[Mitchen]

The injectors may be leaking so it loses pressure when stood. That would explain the long cranking time.

Ben

I’m starting to think this as well. However wouldn’t explain the struggle when it’s idling or when I pull the butterfly and it stumbles and struggles? Or would it.

[BenHar]

The injectors may be leaking so it loses pressure when stood. That would explain the long cranking time.

Ben

I’m starting to think this as well. However wouldn’t explain the struggle when it’s idling or when I pull the butterfly and it stumbles and struggles? Or would it.

It might if they are leaking badly.

Ben

[DanThe]

TPS?

[Mitchen]

TPS?

Potentially as it's one of the few parts I have not changed in the last 10-15 year since I've owned the car, and that's because it's been NLA since I first checked way back in 15-16. Whenever they come up for sale they're old and crusty and people want like £200-300 for one

[DanThe]

First job is to check its working properly, then you can open it up/clean/adjust etc

[BenHar]

TPS normally just needs a good clean.

Ben

[Tzantushka]

I'd also suggest the TPS.
A clean is a good start (and even a small hole for the oil to drain out)
But the best thing I found was a clean/new TPS but also a TPS that was adjusted correctly when the throttle butterfly was closed.

Failing that you may not be getting fuel pressure on cranking due to leaky injectors or fuel pump/filter.
Fuel pressure test would help diagnose that issue - rather than throwing parts at the problem.

For the ease of accessibility in this thread, I'll re-post the information form the late and great Jim Levie out of the USA E30 community.
(his advice has helped me a lot)

Idle or hard starting problems are most commonly caused by intake leaks and/or a sticky or defective Idle Control Valve (ICV). The only reliable method of
locating intake leaks is to have a smoke test run on the intake and crank case and to test the brake booster with a gage and vacuum pump. The complete list
of possible causes of an intake leak is:

• Intake boot
  Throttle body gasket
  ICV hoses & connections
  Brake booster, hoses, and connections
  Crank case breather hose
  Evaporative control hoses, valve, and expansion tank
  Fuel pressure regulator & hose
  Injector seals
  Valve cover gaskets & bungs
  Oil filler cap
  Dip stick o-rings
  Oil return tube o-rings
  Pan gasket or oil level sensor gasket

While leaks in some of those can be found by inspection or by spraying carb cleaner on suspect areas, not finding leaks that way doesn't eliminate the possibility. Only a smoke test will really work.

Once the possibility of intake leaks is eliminated, the ICV needs to be removed and cleaned with carb cleaner until the vane inside moves freely. When the ignition is switched on you should be able to feel vibration from the ICV. If no vibration the ICV is bad, there's a problem with its wiring or connector, there's a problem with the TPS, or the DME (or Idle Control Module (ICM) on an ETA car) is faulty.

For the DME (or ICM) to control idle, the idle switch in the TPS must work correctly. The switch should close when the throttle stop is 0.030-0.060" off the idle stop screw. Hearing a click doesn't mean the TPS is working. Check with a meter or test light.

The fuel system should be tested via the suite of tests in the Bentley manual as invalid rail pressure can be a contributor to idle and starting problems. A simple injector check is to pull the injectors, jumper the fuel relay to run the pump, and see if the injectors are leaking. You can also point the injectors into a towel, remove the coil wire, and crank the engine to see if all of the injectors appear to be spraying in a similar fashion. The best approach to possible injector problems is to have the injectors cleaned and flow tested. Since raw fuel can or will be released in these tests, have a fire extinguisher handy.

While a bad check valve in the high pressure pump can result in longer than normal cranking, if the fuel system is working as it is supposed to the rail will reach normal pressure in a few turns of the engine. A weak pump, clogged filter or leaking FPR in conjunction with a failed check valve can result in longer cranking and/or idle problems.

The O2 sensor can be a contributor to idle problems. The O2 sensor is a scheduled maintenance item with a useful life of no more than 100k. If the sensor has that mileage or more (or is of unknown age), replace it.

The AFM can be a contributor. If the vane doesn't move freely or the resistance track is worn the DME may be receiving invalid data from the AFM. And if someone has fiddled with the bypass air adjustment the DME may be unable to stabilize idle. The bypass air adjustment should only be adjusted per the procedure in the Bentley and with an exhaust gas analyzer. And even then everything else associated with engine management has to first be operating properly. If the AFM becomes a suspect, replacement with a good used unit is the best approach.

Improperly adjusted or malfunctioning valves will affect idle and starting. As can compression issues from ring or cylinder wear. A valve adjustment is called for every 15k. A useful diagnostic is to run compression (dry and wet) and leak down tests on the engine. Aged ignition wires, plugs, distributor cap, or rotor can cause problems. Insulation does break down with time and
heat. And since the youngest E30 is going on 20 years old, if the ignition system is original or the plugs are old replacement is indicated.

Although not usually a problem, a bad DME temp sensor is a possibility. That generally won't cause an unstable idle, but can cause hard cold or hard hot starts and/or a rough idle. As can problems with the timing reference sensors. Although not commonly encountered, a bad harmonic balancer on an M20B25 or M30 engine will cause problems.

When all other possibilities have been eliminated and idle or starting problems persist, replacement of the DME, or if applicable the ICM, is indicated.

In many cases the cause of problems will be a combination of factors. So it is important to test and repair all of the possible causes.

[Mitchen]

Thanks for this guys. I’m going to take the TPS off and clean it. I’m assuming I should take it apart rather than just spray contact cleaner on the pins lol

[DanThe]

Before you remove it, test that it is doing what it is supposed to, otherwise you will never know once its off and in bits. All you need is a continuity tester, im sure the procedure will be in the wiki

[fusilier]

Have you considered testing the Fuel pressure regulator , a simple test with a pressure gauge, fitted as close to the fuel line intake at the fuel rail, where with the engine running you should get a reading of 3 psi slighting increasing when you blip the throttle then return back to 3 when released . if its below that figure restrict the return hose from the FPR and you should see the pressure increase to 5 bar , f so then the FPR is faulty. Taken from advice by the great BrianMoore.

[HartgeH27]

Great reply !!