1990 325i Conv badly lurching under acceleration

[chris_k80]

Hi everyone,

I'm new on here as I've recently bought a 1990 e30 325i convertible after some years of looking for the right one. Its Dolphin grey with black leather and I love it to bits.

Unfortunately it seems to have an engine issue. When driving off, when the engine reaches 2000 - 2500rpm in any gear, the car suffers a complete loss of power. The power comes back after a second or so, but then once the revs pick up again, the power goes again, and so on. If you keep your foot down, the car lurches back and forward like this every second or so and the revs won't climb. Its not like a misfire, the whole engine stops firing completely and the deceleration almost throws you out of your seat. Also its not an intermittent issue, it happens every time.

It seems to get even worse once the car's warmed up, to the point where it won't do more than 50mph in 5th gear, and even breathing on the accelerator above 1500rpm or so causes the loss of power.

The car had been laid up for around 5 years before I bought it, so I changed the following before I got it MOT'd:

New distributor rotor arm
New distributor cap
New spark plugs
New fuel filter
Put 10L fresh fuel in (tank was virtually empty when it left storage)
Swapped ECU (my car had a Motronic 0261 200 380, the swap was to a 0261 200 173 but the Wiki says this should be OK, - symptoms were identical anyway)

Idle is lumpy from cold but stabilises once warm, and the car does not stall. It will rev freely in neutral when cold but even this becomes a problem once fully warm.

I'm guessing the next step is to try getting the injectors cleaned, but does anyone recognise these symptoms of such total power loss, and can think of anything else? If the injector clean fails I think I'll just have to admit defeat and chuck it into a garage as I'm running out of ideas.

Any help appreciated - many thanks

Chris

[Brianmoooore]

What, exactly, does the tacho. do when it cuts out?

[jimbom30cab]

Loss of fuel pressure or dme failure?

Depends on the tacho

[chris_k80]

The tacho shows what I think is the actual engine speed. It doesn't flick about or drop to zero. it just slowly decreases what its showing when there's no power and the car itself is actually slowing down. Then increases again as the car picks up etc.

[Brianmoooore]

The tacho shows what I think is the actual engine speed. It doesn't flick about or drop to zero. it just slowly decreases what its showing when there's no power and the car itself is actually slowing down. Then increases again as the car picks up etc.

That eliminates problems with the crank sensor and the supplies to the ECU., including any immobiliser connected in the standard way.
You've fitted a new rotor arm and cap, so that should eliminate those.
The section of the DME relay that feeds the injectors is suspect, so you could try replacing that if you have one to hand, or replacing it with a couple of wire links between pin 30 and both pins 87 of its base as a test.
Other possibilities (without actually seeing the symptoms in person) are a faulty AFM (see wiki for testing procedure with a battery) or even the fuel pump acting up, giving a drop in fuel pressure.
I see you mention a change in the symptoms as the car warms up, so you should check the colour of the spark plug tips, and check the resistance of the coolant temperature (blue) sensor at the engine ECU plug, with the engine stone cold and then fully warmed up.

[chris_k80]

Thanks guys, will check all the above over the weekend and report back.

[Brianmoooore]

As above ^^ and..

C191 plug under the intake manifold.. check it is not corroded. it links the injectors and the ecu temp sensor to the ecu and can cause all sorts of running issues if it is not connecting the wires properly.

this mf...

This was to be my next step AFTER you have posted the results of the temp. sensor tests. Don't even go near this connector until you have done that. It's important that nothing is disturbed in that area at this stage.

[chris_k80]

Did the first few bits today:

Pulled spark plugs to check them (apologies for blurry pic of Cyl 4):



Also did a compression test whilst I had the plugs out, values were the following: 168, 170, 166, 176, 175, 174

Then I checked the blue temp sensor by the suggested method, via ECU connector Pin 45 and using Pin 2 as ground:



Reading was 2950 Ohms engine cold, 270 Ohms engine warm.

I'll be attempting the DME relay shorting tomorrow, does any of the above info shed any light in the meantime? Many thanks

[chris_k80]

I did the DME relay test today by shorting it with two wires from Pin 30 to the 2 x Pin 87's, this didn't improve the symptoms.

I also checked the AFM resistance, I know the proper way is to do a voltage check but I don't have the bits to do that at present. Resistance was 3240 Ohms static, progressively & smoothly ranging to around 4000 Ohms with door fully open:



I also saw in the service history that the AFM was completely renewed in 2003.

I also tested the IAT sensor which read 3600 Ohms cold, then after around a minute of blowing a hairdryer into it, reading was 1600 Ohms and still dropping.

Having driven the car more it is definitely engine temperature related. When the car is cold its quite driveable, just has a stutter as you go through the revs, but after 15-20mins of driving you can barely get any power at all.

[chris_k80]

Hi, can anyone help me with any further suggestions please?

Today I measured the fuel pressure just after the pump, but it remains constant at approx 3 bar even when the loss of power occurs.



When the car's really warm, it won't even rev past 3000rpm in neutral if you floor the accelerator.

Could it be a vacuum leak somewhere?

Many thanks....

[Brianmoooore]

Not a good place to measure the fuel pressure, since you are before the filter. Should be measured by disconnecting the feed hose at the fuel rail, connecting that to your tee piece, and the hose from that to the rail.
If you're certain that it's temperature related, it still points towards an excessively rich mixture, caused by 'blue' temp sensor or C191 problems, in spite of your reasonable test results, although the appearance of your spark plugs doesn't support this.
Try measuring the sensor hot again at the ECU, and wriggling the wires at the C191 as you do so. Be careful with those meter probes into the ECU plug - they can cause problems if they open up the buckets in the plug too much.

[chris_k80]

OK that's a fair point. I was at least able to eliminate the fuel pump as a potential cause

How's this:



Pressure stays at 3 bar even when stomping it (in neutral).

Getting dark now so I'll continue the blue sensor testing tomorrow.

[Brianmoooore]

If it stays exactly at 3 bar, then there's something wrong with the vacuum pipe to the pressure regulator. The fuel pressure should be 3 bar above the pressure inside the inlet manifold, and with the throttle closed, this should be considerably less than atmospheric pressure, so the gauge should read correspondingly less as well. When you stomp the pedal, as you put it, the pressure inside the inlet manifold jumps up to much nearer atmospheric, which should also be reflected in the gauge reading.
This will probably turn out to be a leak in the vacuum pipe, but I doubt if this is much to do with your main problem.

[chris_k80]

Sorry for the inaccurate reporting - the reading did drop when I lifted off the throttle. Somewhere to around 1.5 - 2 bar I think from memory.

[chris_k80]

In case anyone's interested, this problem is now fixed.

Earlier today I was driving the car to a friend so he could have a look at it. I was taking it easy, and trying not to provoke the issue by accelerating hard. All of a sudden the car lost all power, even though I was only cruising on a flat road and not putting my foot down. Then after a few seconds, it suddenly picked up, and was able to rev cleanly again. After a couple more coughs under acceleration over the next few minutes, it now pulls hard and cleanly revs right round to the red line.

Having gauged opinion, we think the cause of the problem was a blockage in the fuel line which eventually dislodged and worked its way through the system. So I'd advise anyone with a similar issue to flush their fuel line through, which seemed to work for me.

Thanks for the advice and support, which helped me get to the solution in the end!

[Mitchen]

Congrats on the fix