Zetec running a bit rough

[z7]

Apologies in advance if some of these are really dumb questions. I have searched Blatchat, read the Haynes Weber manual, and DV Andrews document but I'm still not clear.

 

I have a 2ltr Zetec with 45DCOE's and a Dunnell 3D ECU. Which I noticed a few weeks ago was starting to run a little rough throughout the throttle range.

 

I noted that, even when warm, the idle revs. had dropped from around 800-1000rpm to 250-500rpm. This lead me to suspect the carbs. may need adjusting. They had last been professionally adjusted about 5 months ago.

 

I've taken a look at the sparkplugs and all of them are sooty (no not the puppet!) implying a too rich mixture. But from threads on here, that isn't a good check any more(?)

 

The extent of my previous carb knowledge is adjusting an idle screw on a single unit.

 

My queries are:

 

1 - is it usual for the carbs to need adjusting periodically and, if so, how often?

 

2 - how does one test the carbs? I know of checking the air-intake by listening or use of a synchroniser(?) But how does one know if all the other adjustments need adjusting?

 

3 - if the carbs are the cause, how do I pinpoint the fault, other than trial and error?

 

4 - what is the interaction between the ECU and the fuel system?

 

5 - am I barking up the wrong tree altogether?

 

Any help would be appreciated. This is all new to me.

 

Adil

Now will you believe I'm a Novicesmile.gif

 

Edited by - adil on 22 Feb 2002 18:09:53

[Peter Carmichael]

I can't answer all your questions.

 

Carb balancing will affect idle mostly, but won't cause too much of an issue throughout the rest of the rev range.

 

The Dunnell ECU will only be as good as its sensors and Dunnell's approach makes it difficult to troubleshoot a set up (no cables, no maps). Check all the sensor wiring. 250-500 rpm is a very low idle.

 

It could be the carbs, but...

[millsn]

It sound to me like your idle jets are set too rich. It is likely that your professional rejet set them this way to compensate for the (assumption) poor progression that you enerally get with 45s on a zetec. After a while the plugs coat up (sooty) and give you poor running (most running is done on the idles - the mains are raely used).

Idles don't normally go out but carb balance can do quite frequently. Also possible is that the carbs are pulling in air from somewhere - I assume you can't find any evience of this around the seals to the manifold?

One last thing - I had this problem and it was my exhaust which had cracked at the flanges where it attaches to the 'head. Just check you don't have this too.

 

Unlikely but it might be your plugs - are they the correct type and are they gapped correctly?

 

The ECU only looks at throttle opening and rpm and adjusts timing accordingly. It is possible to reset the ECU for the throttle stroke I assume you cannot have done this by mistake and anyway the Dunnell ECU should learn its way out of this.

 

Last thing I'd check is that your battery is ok.

[z7]

Peter, Nigel thanks for taking the time to reply. Unfortunately, I haven't had the chance I'd hoped for to have a further look this weekend.

 

Peter, I do actually have a lead and a Dunnell diagnostic disc. It's only one floppy disc so I don't know how sophisticated it is. Or needs to be. I'm trying to borrow a portable from work to see what I see.

 

Nigel, interesting and surprising, to me, that most running is done on the idles!

 

I haven't touched the ECU but how long should it take for it to learn its way out? And I presume this isn't really that good if it's just compensating for a problem?

 

Thanks

Adil

[felix.klauser]

If the car was running OK and then deteriorated over time (which I think is what you're saying) then it's unlikely that the jetting is wrong unless someone's changed it without your knowing!

 

I had the same setup (twin Weber 45's w/Dunnell 3D ignition) and when the idle was rough it was invariably the balance between the carbs that had come adrift.

 

Also check the gaskets between the carbs and the manifold to ensure that they're not leaking.

 

I assume you've checked the obvious like the state of the plugs, their gaps and the leads themselves...

 

Felix

[z7]

Felix, yes she was running okay and has deteriorated. However, perhaps I should explain that the previous owner had had it tuned on a rolling-road such that at idle she needed constant throttle attention. This made it very undrivable for me. So, it was professionally tweaked. But, as it was kind of in passing, perhaps all that was done was to make her run richer at idle. I don't know. Perhaps that was all that was needed.

 

Anyway, the plugs are sooty and so may well be adding if not causing the problem. But then that will be a symptom of a too rich mixture.

 

Thanks for your other suggestions, too.

Adil

[felix.klauser]

I'm no longer certain but I think I used to run 50F6 idle jets, 170 air correctors and 135 mains when I had the engine in standard tune. Hope this helps.

[z7]

Felix, not really but thanks anyway. I'm still trying to get to grips with their function and adjustment.confused.gif To keep things simple for the moment, I've assumed them to be correct and not concerned myself with them yet. I don't want to be removing them to check just yet in case I completely mess things up.

 

Cheers

Adil