E30 325i wont go above 4000RPM!!!

[onthames]

First post, so forgive me if I fk something up.
I have an 88' UK spec E30 325i.
Here are the symptoms of the problem:
When I first take the car out, and put my foot to the floor in say 2nd/3rd, its fine, can hit redline.
Then, after a few runs, it'll refuse to go above 4k, or sometimes 5k (seems to depend on how long I've been driving).
However, when it refuses to go above 4k, this is when I'm keeping the throttle wide open. If I go to 50% I can get it past the limit I'm facing.
BUT, in 5th gear, I cannot get past 4k RPM no matter how light I am on the gas. Just wont go.
So, in short, first few runs no problems, then the car starts to stop accelerating when I'm revving high. I tried pulling over today and giving it a break for 10 mins and trying again, this let me get to around 4.5k whereas the run before the break I couldn't get above 4k.

Replaced fuel pump and filter because I thought it might be that. No luck.

Anyone faced this problem? I feel like its temp related or theres a problem with fuel delivery. But I recently replaced the intank pump and the filter under the car.

Video of it happening.
https://streamable.com/e/n20q9g

Edit: I should add, apart from this, the car runs and idles perfectly. No problems whatsoever and no noises once it hits it's limit, just seems to try and then gets stopped by something, then try again, then stop again, no odd noises while this is happening

Edited by onthames on Thursday 17th June 22:07

[helix402]

onthames said:

First post, so forgive me if I fk something up.
I have an 88' UK spec E30 325i.
Here are the symptoms of the problem:
When I first take the car out, and put my foot to the floor in say 2nd/3rd, its fine, can hit redline.
Then, after a few runs, it'll refuse to go above 4k, or sometimes 5k (seems to depend on how long I've been driving).
However, when it refuses to go above 4k, this is when I'm keeping the throttle wide open. If I go to 50% I can get it past the limit I'm facing.
BUT, in 5th gear, I cannot get past 4k RPM no matter how light I am on the gas. Just wont go.
So, in short, first few runs no problems, then the car starts to stop accelerating when I'm revving high. I tried pulling over today and giving it a break for 10 mins and trying again, this let me get to around 4.5k whereas the run before the break I couldn't get above 4k.

Replaced fuel pump and filter because I thought it might be that. No luck.

Anyone faced this problem? I feel like its temp related or theres a problem with fuel delivery. But I recently replaced the intank pump and the filter under the car.

Video of it happening.
https://streamable.com/e/n20q9g

I once fixed an E30 with a similar problem, it had water in the DME due to a screen leak.

[onthames]

helix402 said:

I once fixed an E30 with a similar problem, it had water in the DME due to a screen leak.

What like a leak in one of the screenwash valves? Cos I had one leak recently... Could be that.

[helix402]

onthames said:

What like a leak in one of the screenwash valves? Cos I had one leak recently... Could be that.

A leak past the screen seal into the DME.

[onthames]

I see, I'll check tomorrow. Well the car runs perfect apart from this issue do I suspect ecu is fine but never know

[Krikkit]

I had a similar issue (although with another car) and it was the cam sensor. I know the M20 doesn't have one, but it could be crank - sensor goes noisy at higher RPM depending on the heat as well.

[E-bmw]

My best guess would be an air leak into the inlet near to the intake manifold.

Theory being when cold the fuel ratio is higher due to cold start enrichening.

When this ceases the mixture is actually over-lean giving the issue you describe.

Can't remember if your engine has concertina type rubber inlet hosing, but these are a prime candidate for this & can be hard to see if the split is at the bottom of the rubber hose "creases", issue could be caused by any intake air leak if not on these type of hoses.

[buggalugs]

As above, the black rubber air inlet tubing around the maf and throttle body splits in the concertina.

[onthames]

Disconnected boot from maf and blew smoke in at 2psi, no leaks as far as I can tell.

[E-bmw]

Daft though this may sound blowing it up may close leaks that are obvious under suction.

Try the old squirt brake cleaner (or similar) trick at the area when it is running to see if the idle speed changes when it is squirted.

A few revs could help in your case.

[onthames]

Will try that. I did find a missing relay when cleaning all the connectors near the rubber boot. Any idea what should be here? if anything.

[helix402]

onthames said:

Will try that. I did find a missing relay when cleaning all the connectors near the rubber boot. Any idea what should be here? if anything.

I suspect the missing relay is for an option your car doesn’t have, such as a cat. Re the white relay-I can’t remember if it’s DME or fuel pump, I can remember that they are renowned for failing (won’t be the cause of your 4K fault IMHO). As the relay is original in the interests of reliability I would recommend replacing it with a brand new gen BMW one.

[onthames]

helix402 said:

I suspect the missing relay is for an option your car doesn’t have, such as a cat. Re the white relay-I can’t remember if it’s DME or fuel pump, I can remember that they are renowned for failing (won’t be the cause of your 4K fault IMHO). As the relay is original in the interests of reliability I would recommend replacing it with a brand new gen BMW one.

I think its the O2 sensor heater relay. I'm not sure if it is causing my problem but I think there should be a relay there... Will put one in and see if problem goes away, unlikely it will but would be a cheap fix!

[onthames]

I'm trying to figure out the problem before I start replacing bits and bobs as could get costly pretty quickly.
The main thing I'm trying to work off is that this starts to happen after about 15 mins of driving. So, in that time, what changes? I'm pretty new to this (this is the 1st car I've ever worked on!) but the only thing I can think of would be temperature, pressures and richness?
Which leads me to believe it may be a sensor issue?

Any thoughts on that theory? Or am I using too much logic for this old old engine!

[Krikkit]

Temperature soak into all components. Commonly induces issues in electrical items which are flaky.

[sunbeam alpine]

One very simple thing to try is running without the fuel filler cap. If the tank isn't venting properly, fuel can't get out. Produces these kind of symptoms.

[onthames]

sunbeam alpine said:

One very simple thing to try is running without the fuel filler cap. If the tank isn't venting properly, fuel can't get out. Produces these kind of symptoms.

Am I okay to drive with it like that to test?
not sure if e30s have flapper valves

Edited by onthames on Friday 18th June 23:21

[E-bmw]

Another potential issue is the coil.

These are single coil only & pretty much any ignition coil will do the job at least to try to check the issue out.

I have had them break down under heating before.

Visit ebay & you will likely get one for peanuts that will do the job.

[sunbeam alpine]

onthames said:

sunbeam alpine said:

One very simple thing to try is running without the fuel filler cap. If the tank isn't venting properly, fuel can't get out. Produces these kind of symptoms.

Am I okay to drive with it like that to test?
not sure if e30s have flapper valves

Edited by onthames on Friday 18th June 23:21

Fine to drive like that to test the theory. Probably best not to do it with a full tank.

[onthames]

Gonna try it today. Also gonna test the "blue temp sensor" with my multimeter following a guide on e30zone. Will report back