Ridiculous Computer Problem

[llll1l1ll]

Hello all!

'88 325is here.

Backstory: thought it was the injectors or something along those lines; however recent events lead me to believe otherwise.

Problem: unnecessarily large amount of fuel delivery to number four cylinder - so much to the point that it will not ignite causing it to spew fuel out the exhaust (wtf!) and run terribly

Current solution: unplug battery, wait 10 minutes, plug back in, drive away as if nothing happened - three days later, though, it comes back - simply disconnecting the battery sovles it all

What is the deal? I've replaced the CPS. The only two things I could think of would be the O2 and the throttle position sensor.

If it wouldn't be those two things, then what could it be? The main relay? The main brain itself?

Any help is greatly appreciated as it looks as if time is limited for my little Betty.

Best,

Peter

[nando]

does your car have a 153 ecu? if so, I'd swap it for a 173 and see what happens.

[llll1l1ll]

Y'know what? I'll double check my ECU number. I should have looked at it.

[nando]

it's probably a 153 since you have an '88. functionally the same as a 173, but the hardware itself is a lot more complicated (dual motherboards, etc) and prone to failure.

[jlevie]

Understand that the injectors are organized and fired in two banks of three. Bank 1 is cylinders 1,3,5 and Bank 2 is cylinders 2,4,6. So a DME problem can't cause an excess fuel delivery to only one cylinder. That has to be an injector problem.

The observation that the car will run better after clearing the DME's adaption suggests an aged O2 sensor, intake leaks, or a bad AFM.

What I recommend is:

1) Check the rail fuel pressure to see if you have a bad FPR or obstructed fuel return line causing excessive rail pressure

2) Pull the injectors and have them cleaned and flow tested

3) The useful life of the O2 sensor is not more than 100k. If that applies, replace the sensor.

4) Have the intake smoke tested and repair any/all leaks found.

If the problem persists and clearing the adaption continues to help, replace the AFM and then the DME (in that order).

[llll1l1ll]

Thanks for the great response!

I will try the O2 sensor first as it is stock.

I'm trying to avoid pulling the injectors since I will need to detatch the intake manifold, which is a fairly large amount of work thanks to that oil tube.

[nando]

you don't need to detatch the intake manifold to remove the injectors!

Drain pressure from the fuel rail (pull FP relay and start the car, let it run until it dies). remove the valve cover (for access clearance) and the manifold brace. Undo injector harness plugs. Undo fuel rail clips and 10mm bolts, and fuel lines. Remove and clean.. maybe have them rebuilt professionally if the downtime doesn't matter (I use www.cruizinpeformance.com).

late cars are even easier, since the injector harness can seperate from the main harness, and you just pull up to disconnect it from the injectors.

[jlevie]

There is no reason to remove the intake manifold in order to remove the injectors. You'll want to remove the valve cover to gain better access to the fuel rail. So you'll need a new gasket & rubber bugs. If it hasn't been done in a while (in the last 10k) valve adjustments would be good to do while the cover is off.

[llll1l1ll]

The FP relay is fuse 11, no?

I tried pulling them the other day. I disconnected the fuel rail hose, then the 10mm bolts, then the wiring plugs for the injectors, but when I pulled up on the rail nothing would budge or come out. I didn't want to break it so I didn't go any further.

Bummer, too, since I figured I could knock it out fairly quickly, and now I have six new injectors hangin' out in my bedroom.

[llll1l1ll]

Went ahead and replaced the O2 sensor. Car runs smoother, but I am suspicious still.

Also, it should be noted that I have the 153 ECU/DME.

[nando]

definitely consider a 173. they often sell for about $35.

[llll1l1ll]

Seriously? I was dreading a $200+ price tag.

So far, no problems yet. Car is acting normal.

[jlevie]

The only advantages that a 173 DME has over a 153 or later are:

It may improve the idle
It supports the stomp test for diagnostic codes
Chips are available for it

The DME's are otherwise equivalent. Well, I have seen 153 DMEs make more power than a 173 as tested on a dyno.

[llll1l1ll]

Well, I don't think I'll notice the power difference that much between the two.

I did not know that the 173 does not support stomp testing and cannot have chips...

[jlevie]

I had the DME types switched (now fixed). It is the 153 that doesn't support the stop test, etc.