Running rough (on couple of cilinders) on cold start

[E30hofmeister]

Hi,

First of all, great forum, have enjoyed reading many threads already :)
Decided to post a question to the m20 experts on here though as my m20b25 runs ok after a minute but the first minute after a cold start it runs horrible.
It starts right up but seems to be running on 4 cilinders or something, splutters a bit as well if I give it some gas.
It sounds horrible. It only takes like 30sec to 1 minute, the time it takes to drive out the garage let’s say, and afterwards it’s fine, but it is annoying
Changed so far:
- new spark plugs (bosch)
- new distributor cap and rotor
(- others: oil (have put in 20w50 valvoline vr1 for the zinc) & filter, cam belt, water pump, non-worn standard cam)
The issue in not present after a hot start, only on cold start.

Hope this sounds familiar to someone and you can point me in the right direction to fix it.
I have worked on engines before, just not too familiar with the m20 yet :)
Is there an additional cold start injector or something, like on vw engines from the same era?

Thank you for helping :)

[E30hofmeister]

Ok from what i’ve read so far my car doesn’t have a cold start injector as it’s a late 1986 325i. The ecu simply runs the mixture richer via the 6 normal injectors.
Could be temp sensor then not telling ecu it’s stone cold...?
Which sensor do i check for this?

But i dont feel too sure about this diagnosis since it does start right up...
Checked Bentley manual on the issue as well, but it’s not that helpful because it basically lists nearly every ignition and fuel component and all kinds of possible vacuum leaks :)

So was hoping the symptoms i describe would sound familiar to you guys and you could offer some advice on where to look first :)

[Todd Black 88]

Injectors leaking on a couple cylinders

[e30davie]

Checked the ECU temperature sensor?

Does it drop the same cylinders every time? as in if you pull the spark plug lead on that cylinder is it clear that is not firing? Could be leaky valve if the case.

But if its generally just running rough (not a specific cylinder) id be looking at the temperature sensor first. TEST IT and then if its stuffed replace it.

[citizen_insane]

Checked the ECU temperature sensor?

Does it drop the same cylinders every time? as in if you pull the spark plug lead on that cylinder is it clear that is not firing? Could be leaky valve if the case.

But if its generally just running rough (not a specific cylinder) id be looking at the temperature sensor first. TEST IT and then if its stuffed replace it.

Agreed. That temp sensor is really easy to test. Pull it, measure resistance, put it in ice water, measure again, put it in boiling water, measure a third time. Should give you a curve you can compare against the spec.

[E30hofmeister]

Thanks for the advice! Will start there and check the temp sensor. That would be the blue one and not the brown one, correct?

[E30hofmeister]

The blue temp sensor seems fine if i compare it to the resistance values in the Bentley repair manual: around 400ohm when hot (5-10 minutes after shutting off warm engine) over several reasonable values in between down to 3350ohm fully cold just now (it’s around 7 degrees Celsius here (=45 Fahrenheit))

E30davie: could you explain what exactly you mean with leaky valve?

Todd Black 88: is there an easy way to check for leaking injector?

[TobyB]

Are you losing any coolant?

t

[E30hofmeister]

Don’t think i’m losing coolant: haven’t driven it all that much since I got the car (and did cambelt, water pump, coolant around 200 miles ago) but looks to be at the same level in the expansion tank.
What is the reasoning behind your question: water leaking via head gasket into couple of cilinders? Interesting suggestion, but would it also not have a hard time running if it sat 30’ with a hot engine for example?

[TobyB]

Yeah, what can happen is that the gasket will only leak dead cold.
So you shut down, the engine cools, there's some residual
pressure in the cooling system, and a cylinder or 2 gets wet.

On the next restart, it takes a while to blow those clean before they'll fire.
It's not enough to notice the loss if you're not really looking- just takes a tiny bit
to ground the plug, and make it misfire.

I've had it happen several times- once I learned what to look for, a borescope was
the only surefire way to find those little green (yes, green, I don't buy the blue BMW myth)
droplets. The leaking cylinders, in my case, weren't even cleaned up like a traditional
head gasket leak.

But honestly, a leaking injector's quite a bit more likely. If you've ruled everything else
out, then maybe start looking at the head gasket.

hth

t

[E30hofmeister]

Ok, thanks :)
Here is an idea: would i be able to test this scenario by releasing the pressure of the cooling system by SLOWLY (of course) opening the cap of the expansion tank after shutting off the engine when hot, and then checking the next day with the engine completely cold again whether or not i have the same cold running issue?

[TobyB]

Yeah, that might do it.
Or have someone else start the car while you sniff the exhaust. It's not GOOD for you
but you can often smell the antifreeze as the car blows it out the tailpipe.

t

[E30hofmeister]

Did the test (release the pressure of the coolant system on hot engine, then check cold running after >6 hours switched off), but had the same cold running issues.
Also don’t notice distinct smell out of exhaust or coolant level dropping.
So leaking past the head gasket is probably not the cause of the cold running issue.
On this cold running engine, i first unplugged the ICV, engine ran worse and nearly died. Plugged it back in, then did the same with AFM, unplugging that also made it run worse and then die. So those probably aren’t the culprits.

Had a good look around, found the spark plug wire on cilinder 6 had a crack in the isolation (not too bad but still) and they look pretty old so considering I already replaced rotor, cap (which were pitted) and plugs, might as well bite the bullet and also replace all the leads.
Measured resistence on the coil and got 1,5 ohms on the primary circuit rather than the 0,5 ohms Bentley manual quotes.
So i guess I’ll replace that as well, or is it ‘within margin’? My multimeter is pretty cheap...)

Also noticed 2 slight cracks (don’t think it’s all the way through though) in the intake rubber boot, see attached picture (hope it’s sufficiently visible). Should it be replaced?

Also paid a bit more attention to the poor running cold engine and actually besides the symptoms i mentioned one other thing it does is rise revs, lower revs, rise, lower, etc.

[e30davie]

there must be ZERO air leaks between the AFM and the engine. So yep, replace that rubber hose.