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Old 05-21-2019, 09:12 AM   #1
S.J.1981
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I may have a very slight misfire, or not?

It has been since mid January since i got the car to be running properly.

I say properly, because since the first day i got it, i know it has vacuum leak(s) and various other small issues.

Even the valves were incorrectly adjusted, which masks the sound and performance of the engine.

Saturday i finally replaced the fuel pump, which turned out to be the cause of my start, fire and dies issue despite almost instant 3 bar pressure at the rail.

Basically, everything except the AFM and the O2 heater relay have been replaced, injectors included. I feel, that there is a tiny misfire.

Today i pulled the plugs, Bosch W8 ones and they are bone dry (no oil anywhere) and a very nice relief to see brown/tan colour on all of them, yes gapped to 0.7mm (0.8mm feeler gauge does not fit)

As of Saturday, the entire ignition system, fuel system and cooling system have been refreshed (cap, rotor, coil, pump, radiator, CPS sensor, wires w/sensor, belts, tps etc). So i never actually heard this engine run well before, and this maybe in my head.

There are no idle issues cold or warm, no start up issues (warm or cold), no acceleration/off throttle issues whatsoever. The engine is super smooth when driving, be it cruising, 1/2 throttle or WOT.

The coil was the only thing i tested that read stranger resistances. I saw 0.8 ohms between terminals 1 and 15 as well as 6,750 between 15 and center. Which is indeed high but it is a new coil, is this normal?

Testing the fuel system, i checked the flow and recoded 1,100ml in 30 seconds (brand new pump) instead of the 875ml in the manual.

I vacuum tested via a cold start cleaner all over the intake manifold, injectors, TB, AFM, boot etc. No issues found, so i did a quick smoke test, by blowing smoke into the intake via the vacuum lines for the FPS. I had someone in the shop check with a flashlight, we both could not find any smoke escaping. Only when i applied compressed air into that vacuum line, smoke started to come out of the air filter box (some smoke, not all of it).

So i take it that there are no vacuum leaks, even checked the DME pins and verified what the sensors have, everything is spot on including grounds.

Could that slightly higher resistance coil or slighter more flowing fuel pump be the cause of this slight (very slight) misfire? or is it in my head?

As i said, there are no issues at all, when driving or starting the car. Just idle sounds like...well an old BMW
but maybe with carbs instead of injectors? i watched several "misfire" videos on youtube, mine sounds nothing like what is online.

I do have the old coil, the original one from 1990, i need to test the specs but i think i replaced the coil just for insurance so it does not fail when travelling across germany or something.
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Old 05-22-2019, 05:44 AM   #2
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It definitely is a small misfire/rough running.

Found the old coil, 1 and 15 shows 0.8 ohms and 15 center shows 6,000 Ohms, still higher that spec. Installed and made no difference, though it feels less punchy now compared to the newer coil.

I shall swap the Vemco TPS with a Bosch one, and double check the idle screw and adjustment to be on the safe side.

Also picked up some NGK V15 plugs, heard the M20 likes to run them over Bosch ones, will report back.
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Old 05-24-2019, 11:59 AM   #3
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Have you ran a compression test or leak down test to see if all cylinders are within spec? If you have a lazer thermometer, you could see if each header run is close to the same temp. That could help identify the cylinder.
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Old 05-25-2019, 08:20 AM   #4
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Quote:
Originally Posted by jbontke View Post
Have you ran a compression test or leak down test to see if all cylinders are within spec? If you have a lazer thermometer, you could see if each header run is close to the same temp. That could help identify the cylinder.
Why would i run a leak test? i do not have any cylinder head gasket issues, no smoke, no loss of coolant or even oil. Unless you mean a different test?

I just replaced all Bosch plugs with NGK ones, double checked the ignition system from scratch and all is good.

Also replaced the Veco TPS with the Bosch one, double checked throttle plate and TPS adjustment, all good.

Still misfires, either there is a vacuum leak i can not detected (such as the oil return tube under the intake) or a fueling issue (FPR most likely since pump and injectors are new).

I can tested the header temperature as you described though.

I have new intake gaskets and tube o-rings, but that first intake manifold nut is being blocked by a cooling line and i really wish not to drain the cooling system.
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Old 05-25-2019, 11:16 AM   #5
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If I remember correctly, its impossible to remove the intake manifold without removing the thermostat housing. You probably wouldn’t need to drain all the coolant, just whatever is in the housing.

If you have spares, you could swap the DME just for kicks. There may be value in disconnecting the battery for an hour or so to reset the flash memory in the fuel maps. I’m not an expert but I think I remember something about this reset.

The compression test is to diagnose a leaky valve which could make a funny sound in the exhaust, depending on the valve.

The vac line going to the brake booster was the bugger that got me for a while, make sure there’s a good seal and that the back pressure valve is operating.
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Old 05-25-2019, 02:46 PM   #6
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Quote:
Originally Posted by 325e '87 View Post
If I remember correctly, its impossible to remove the intake manifold without removing the thermostat housing. You probably wouldn’t need to drain all the coolant, just whatever is in the housing.

If you have spares, you could swap the DME just for kicks. There may be value in disconnecting the battery for an hour or so to reset the flash memory in the fuel maps. I’m not an expert but I think I remember something about this reset.

The compression test is to diagnose a leaky valve which could make a funny sound in the exhaust, depending on the valve.

The vac line going to the brake booster was the bugger that got me for a while, make sure there’s a good seal and that the back pressure valve is operating.
Looking closely, i can remove the intake without touching the housing, but that hose would surely need to come off.

I shall try the battery disconnect trick and see.

Most likely one or more valve are a tad "loose" when i did the adjustment.

All hoses are new, including the brake booster hose and the check valve, but will check and make certain of that hose.

Other that either a slowly dying FPR or a small leak somewhere, everything else looks good.
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Old 05-25-2019, 03:41 PM   #7
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A leak down test is where you bring each cylinder to TDC and add air pressure to the top of the cylinder via spark plug. Leak down gauges can tell you the % loss of air, and listening to the intake, exhaust, or crank case you can determine where the engine is worn. I have seen cars run well but with a slight misfire because of poorly seating valves. The cylinder pressure was enough to push the valve closed under hard pulls to not be noticeable, but under light load you could feel it, but not put your finger on why
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Old 05-26-2019, 02:50 AM   #8
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I am still unsure why you are recommending leak down and compression tests, when there still maybe a vacuum leak from the bitch tube, or another source (small leak).

The head has been rebuilt around 4,000 kms ago, and as i stated there are no oil or coolant consumption issues, no smoking issues either.

I do have new intake manifold gaskets as well as the bitch tube o-rings. I think first step is to make absolutely sure everything is truly sealed up nicely.

FPR is holding vacuum and the pump is new, so are the injectors. Maybe i should take a closer look at the cap and rotor, maybe there is a small crack somewhere (despite also being new).

After i addressed all of the above, then i have no choice but to do a leak down test
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Old 05-26-2019, 09:51 PM   #9
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I believe the reason it was suggested is because you have done pretty much everything that would be suggested. You even mention your ignition system has been refreshed(coil, cap, rotor, wires, and plugs), vacuum leaks eliminated, and "no idle issues." It sounds like you have been over everything. Why do you suspect a vacuum leak if idle is stable and rock solid?

Poor valve sealing won't necessarily cause oil or coolant consumption. I suggest that you at least hook up a vacuum gauge to the manifold and post your results. Just because your head was rebuilt doesn't mean it was done correctly. You've already mentioned that your valves were out of adjustment. 4k kms is plenty for a guide to be ruined when sand or metallic particles weren't cleaned out before assembly, for example. In my view, testing is free, replacing parts that aren't bad is not.
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Old 05-26-2019, 10:14 PM   #10
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I like how you asked for advice, yet are ignoring it.

My first M20, cylinder 5 would misfire when cold, but once the engine heated up it came good. Compression was lower on that cylinder but not what i would call catastrophic (120psi vs 150psi was the rest). No loss of coolant or oil. It was a dodgy valve. When i pulled the head the valves were all very rusty, the engine had sat for years before i got it so i think for some reason cylinder 5 was more effected than the rest.

But of course we can only go on your description of your problem, so who knows what is actually going on.. but if you get favourable compression and leak down results. then we can at least rule that. At least do the compression test. the compression test tool is like 30bucks and takes half an hour with a helper.

These maf engines do not like vacuum leaks at all. even the slightest of leaks will cause it to run shit. Hose clamps that you think are done up tight, you give it a 1/4 more turn and misfires go away......
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Old 05-27-2019, 03:04 AM   #11
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Indeed...

Looking at what you've posted, assuming no minor leaks & given everything else you've done, have you cleaned, and / or replaced your Earths?

Is the one between the head and the strut tower present on your engine + the one on the block? How is the negative lead looking on the battery?

I've had a couple of different M20s with minor misses at idle solved with replacing the negative battery lead + cleaning the earths on the engine. My M30 began struggling a bit this week with the transition from idle, and when I got home I found the negative lead on the battery terminal had developed a crack. Replaced it and all back to normal.
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Old 05-29-2019, 12:32 AM   #12
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On the contrary guys, i very much appreciate any feedback you give. I was merely asking the idea behind the leak and compression tests, since you caught me a bit off guard. I apologise if i indicated in anyway that i do not appreciate or disagree with you, believe me i am not.

Will be checking the grounds, will order a leak down gauge as well, i believe i can fit the fuel pressure kit to act as a compression tester, it did come with various adaptors which may fit.
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Old 05-29-2019, 03:58 AM   #13
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No problem.

Tone can be a bit hard to pick on the web sometimes.
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Old 05-30-2019, 10:18 AM   #14
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Would like to give an update, since it is a holiday here in Germany, i decided to re-check everything once again to be certain.

Still can not find any vacuum leaks after several smoke tests, all sensors/DME pin read outs are within specs.

Fuel pressure (bypassing relay) is just a tad under 3 bar, fuel flow is around 1,000 ml in 30 seconds.

Fuel pressure when engine running is 2.6 bar, which is a bit lower than the manual specs. So i replaced it with a new 3.0bar unit, and it made no difference. Still showing 2.6 bar, no change with engine load (increasing the revs) and goes up to 3.0 bar when the vacuum line is removed.

Did several WOT runs, it pulls and runs just fine, in fact that engine responds really nicely when not on idle. I did have a strange warm start issue the other day, it cranked, fired and died twice (old FPR was installed).

If there was a blockage in the lines, then the fuel pump would also not rear correct pressure nor flow. New FPR is rated 3.0 bar but also gave me the same 2.6bar results, so i am unsure what to make of this now. Fuel filter was replaced around 2,000 kms ago.
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Old 06-11-2019, 12:12 PM   #15
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I apologize for the late update, i had other things t attend to. Replaced the LCAs and bushings last week (offset PU) and handles great.

Today, i decided to rip off the intake manifold. To my suprise one nut was half way threaded in. Most likely is the cause of a vacuum leak (i believe it was where the center gasket goes). Also found a small hole/tear in the breather hose from the TB to valve cover (where the hose clamp sits). All new gaskets, new bitch tube o-rings and new hose. Re-checked the TB stop screw/plate and TPS (it was already out). Lightly lubricated the gaskets with silicone grease (to ensure a seal) and lightly greased all o-rings (including injectors). I did not wish to use any kind of sealant, since i know next time removing any of these gaskets will involve scraping off the old stuff (the silicon grease can be reused or just wiped off).

She idles great now, sounds a lot quieter, a bit more responsive as well. Looks like it was either an intake manifold gasket leak, breather hose leak or any of the above (or a combination). It is strange, since i was the last one to touch the intake, and i always double torque the nuts/bolts to be sure nothing is missed. Must had come loose somehow.

Did not do any WOT pulls yet, just spent 6 hours in the workshop checking suspension torque, flushing p/s fluid and the intake. But so far, all is looking well.
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