Porsche Enthusiasts Club Forum

Hi Guys,

After many forum searches I'm hoping someone might be able to shed any light on the following:

Car: Standard 1988 2.5 8v 944, UK spec, non-cat

Problem: Car seems running pretty rich and has failed the MoT on its emissions:
CO value 4.99% (FAIL, pass is 3.5%), HC count is 229ppm (PASS, 1200ppm max).
Last year's result:
CO value 1.84% (PASS, same garage, same equipment)

Also related:
Plugs, just removed, look like this:

(last year they were light, sandy colour)

Not sure if it's related (probably is), but over the last couple of months (normally when just after starting up when engine's cold/warm) the car has occasionally hesitated while driving or idling (like a misfire, but with a smell of fuel and dark smoke from exhaust). Usually clears up within 10-20 seconds. Has occasionally caused car to stall and stop, but has always (reluctantly) restarted. After this, usually runs fine from then on (no smoke or fuel smell, though does smell a trifle rich)

Actions so far:
- tested fuel pressure regulator as per Clark's guide http://www.clarks-garage.com/shop-manual/fuel-01.htm, readings were as follows:
Engine Stopped - Relay Jumpered = 2.7 bar (borderline pass, should be 2.5 bar +/- 0.2 bar (36 psig +/- 3 psig))
Engine Running at Idle = 2.2 bar (borderline pass, should be 2 bar +/- 0.2 bar (29 psig +/- 3 psig))
I'm going to change this as it's clearly not helping having too high fuel pressure, though I doubt this is the root cause.

- checked readings given by DME temperature sensor as per Clark's guide http://www.clarks-garage.com/shop-manual/elect-19.htm#dme-temp:
DME Temperature Sensor Resistances:
14°C - 22.2 kΩ (seems huge and might explain hesitation when cold, but the emissions test is done with engine at full working temperature)
80°C - 353 Ω (seems to tally with Clark's figures, (then rises slowly as engine cools)

- changed DME relay for a used spare (though this was probably pointless as it either works or doesn't right? Shouldn't cause it to run rich I'd've thought)

- checked some vacuum lines; no obvious leak (sprayed with WD40), but test wasn't exactly thorough / scientific...

- tested spark plug lead resistances - all between 2.97-3.03 kΩ

- tested AFM output as per Clark's guide http://www.clarks-garage.com/shop-manual/elect-22.htm
all readings were ok:
ground terminal (2) on the inside of the AFM and the positive lead to the sensor output voltage terminal (4) = 0.48v (closed) to 8.64v (fully open), seemed pretty proportional all the way.
using new 9v batter (9.58v)

- next signs would point to an O2 sensor, though as this is a non-cat car, I don't think it has one...

Any help appreciated,

Cheers,


Simon

It might be just the photo but those plug gaps look huge?

Yeah - they vary from 0.96mm to 1.04mm. Were all gapped to 0.8mm when I put them in (1 year /10k miles ago).

Hi, I think gn3dr has hit the nail on the head, looks like you've got your decimal point in the wrong place.

You mean the 22.2 kΩ figure is more likely 2.22 kΩ? If so, that's not too far off Clark's 3.3 kΩ figure - or did you mean something else?

When was the oil last changed and did you ever take it out on long journeys or simply short ones. How much does the oil smell of fuel? A good service (oil, plugs gapped correctly and filters (air filter, oil filter and possibly fuel)) may be enough. When were these last done on the car?

For my Stag it certainly was the case with the fail readings being similar to yours, and the plug change (on plugs which only had 4 or 5k on them!) cured an intermittent running issue.

Saying that, it does seem fairly rich. Where the the decimal dot for your coolant temperature (these can throw the emissions off in modern cars). Did you check the IAT sensor within the AFM as well? Besides the odd running issue does the car run smoothly despite being rich?

I meant the spark plug gaps, unless its an optical illusion they look to have an 8mm gap not 0.8

I agree the gap looks massive. Try gapping them correctly or replacing them.

Stuart

Hi Guys, thanks for the replies. In answer:

I think the photo is a bit of an optical illusion! See below pic taken from different angle:

The gaps are a little large (been in a year) not not 8mm (thankfully).

Car has just had new plugs, oil & filter change, air filter clean, no difference. When running ok, engine is very smooth and pulls well.

I've not checked the IAT sensor - is this within the AFM?

Update:
Have just fitted new Fuel Pressure Regulator, readings are better (though I don't think this has solved the issue as the original readings weren't horrendous:
Engine Stopped - Relay Jumpered = 2.5 bar (PASS, should be 2.5 bar +/- 0.2 bar (36 psig +/- 3 psig))
Engine Running at Idle = 2.0 bar (PASS, should be 2 bar +/- 0.2 bar (29 psig +/- 3 psig))

When fitting the new FPR, I noted that removing each vacuum line in turn resulted in an increase in fuel pressure, except the one running from the AFM to a sensor on the back of the head illustrated here:





Where removal of the line resulted in no change to fuel pressure. The line is clear (well, you can blow through it and feel air the other end, at least). This was the same at ambient and approx normal running temp.

Might be of significance?

A vacuum leak would indeed potentially cause rich running. The line you are curious about looks to be connected to throttle vacuum, thus at idle there should be no difference in theory. Someone with 8V N/A knowledge should give a better answer. When you say you cleaned the air filter I assume that you have a K&N air filter?

Yes, the IAT is within the AFM itself, and can be tested as per the Clark's guide.

Thanks for the replies. In answer:

Yes I have tested the AFM's IAT (I thought the IAT was another sensor inside the AFM, but it's just a different name for the barn door / potentiometer, right?). Results were good:
ground terminal (2) on the inside of the AFM and the positive lead to the sensor output voltage terminal (4)
= 0.48v (closed) to 8.64v (fully open).
seemed pretty proportional all the way
using new 9v batter (9.58v)

Yes, car has K&N panel filter (clean).

When I said IAT I meant inlet air temperature sensor, which on the Series 2 cars was put within the AFM itself. This sensor if it has gone bad can also cause rich or lean running.

Yep, I tested that with a 9v battery as per Clark's guide - readings seem bang on.

Update:

Changed distributor cap (points pitted and eroded) and rotor arm and had it re-tested:
CO value 4.6% (FAIL, pass is 3.5%), but a slight improvement perhaps due to new FPR and dist cap.

BUT, after the test the garage let me keep the CO meter in while I fiddled around under the bonnet. It seemed that when I moved the green vacuum hose here I disturbed something and the CO reading dropped suddenly to 2.9% - PASS!

Car MoT'd. Bang.

However, after removing the green, orange lines and surrounding elbows, cleaning them out with carb cleaner and re-running the test, emissions are back up to CO = 4.7% ! No amount of fiddling resulted in a drop of CO value again, meaning it may have just been a coincidence.

Guess next step is to trace all the vacuums as shown here: http://www.clarks-garage.com/shop-manual/fuel-13.htm....but any brighter ideas welcomed!

did you bang or move the earth cable?

Quite possibly!