Replacing DME?

[jlevie]

It isn't necessary to replace the DME power cable, but it may be necessary to replace the fusible link. For that I use a 60A lug type fuse that I bolt in where the DME cable attaches to the power block in the engine bay (and splice out the fusible link).

All of the grounds for the engine mamagement system are at the right front shock tower.

[Jay87]

Replaced my fusible link with a 14ga fusible length from Autozone... old one was extremely crusty. It seems strange to me that the issue would be intermittent., but we'll see if this solves the problem.

[iwantspeed]

Hey guys,

Having an issue where my car will occasionally die briefly at highway speed, with the tack dropping to zero, and pick right back up and continue. Thinking it is either the main relay or the DME. Any way to tell which?

Thanks!
Jay

my car did this

replaced a ton of things
ended up being the ecu
i replaced it with a known good one and problem solved

[Jay87]

Wanting to keep this thread live... here is a video of what the car does. Sorry for the lame narrative.

http://www.youtube.com/watch?v=otnVVdXTfRg

I replaced the DME with a rebuilt unit. We'll see if this fixes the problem... I don't know where else to look.

Refresher, Replaced:
Main Relay
Fuel Pump Relay
DME Fusible Link
Crank Position Sensor
Cleaned DME Power/Ground connections

Now, replaced DME.

Based on how the CEL is blinking, is my old DME going out?

[jlevie]

If the problem persists with the new DME, replace the engine harness.

[Dude]

if the main relay died then more than just the engine would shut off

Not if its a loose/old main relay. Had the same issue. Pulling it out, cleaning the contacts, and pushing it back in fixed it.

[Jay87]

Well, looks like I won't be solving this one. Rings are shot on number four cylinder so bad that it wont fire and I don't think the car is worth the rebuild.

For what its worth... I ran into a BMW tech at a rallycross the other weekend, and he told me to look at the idle air control valve. Said they can swing open introducing so much air that the mixture leans out beyond the ability to combust for a quick second.

He did not seem to have a good idea why this would cause the tach to dive to zero, so I'm not sure I stake much confidence in the theory, but I'm posting it here in the hope that it will help someone.

If anybody knows of a good M20 in West Michigan, I might be interested....

Thanks for all the help so far.

[jlevie]

A/F ration problems, like the ICV case mentioned, can cause the car to loose power. As can a bad spot in the AFM resistance track. But those won't cause the tach to drive toward zero. The only things that will cause that are a loss of power or timing data to the DME

[u3b3rg33k]

For what its worth... I ran into a BMW tech at a rallycross the other weekend, and he told me to look at the idle air control valve. Said they can swing open introducing so much air that the mixture leans out beyond the ability to combust for a quick second.

He did not seem to have a good idea why this would cause the tach to dive to zero, so I'm not sure I stake much confidence in the theory, but I'm posting it here in the hope that it will help someone.

I have to say I completely disagree, and I wouldn't trust that tech to do any work on my car, if that's what he really said.
IF the ICV could move that fast, what would happen when you floored it quickly on your EFI engine?

Unless your ICV goes to atmosphere instead of being plumbed properly (remember, ICV operates on metered air, behind the AFM), a sudden opening of the valve on the freeway would be registered as a slight surge in acceleration. Look at how small the ICV tubing is. There's simply no way that that valve could have more than a marginal effect on an engine under load.

I would bet that with a wide open ICV and a closed throttle plate, you won't be able to move more than 20-25 hp worth of air through it. Less with a partially open throttle plate (less manifold vacuum = less flow through the ICV plumbing). Our H-twin chipper has larger diameter intake plumbing, and it's rated at 18hp.