mixture setting/rich smell?

[rjd2]

Hi all-first off, a MASSIVE thank you to the good people on this forum. You've helped me on numerous occasions; this is an invaluable resource for DMC-12 owners.

I recently completed a thermister replacement job, and now have the car back sorted-no fuel leaks, smooth idle, everything seems to be working properly. The last goal I had before buttoning it all up was to make sure the mixture was set properly. I sealed off the "cabin" side of the vacuum hose at the oil filler cap location, and hooked up my dwell meter. Here's what I get:

1-idles solidly at ~750-800 or so.
2-starts immediately, holds with no goose-ing of throttle.
3-dwell meter holds at ~42-45
4-when the "cold start" phase hands off to the normal hot run phase, I get some idle hunting, one or two WILD swings, and then it stabilizes right around 42-ish. With a hot engine, I have yet to see any idle hunting at all.

I can't seem to shake the feeling it's still running rich, mainly from the smell. I don't expect the car to smell like a 2017 Accord, but the richness smell is intense. If I do a 5 minute test routine, my clothes will smell for the rest of the day. If this is par for the course, then no problem. Should I aim to tune it leaner even if my readings seem to look ideal? Thanks for the advice!

Oh, and to all the folks down south, good luck, and stay safe...

[Bitsyncmaster]

Your dwell setting should be less than the dwell you read on the cold start. Starting the cold engine the system sets the mixture rich so the cold engine will run.

So your dwell on the hot engine should be set to 30 to 35 degrees.

[NckT]

You really need to start with fuel pressure diagnostics first to ensure that the Control Pressure Regulator (CPR/ WUR) is operating within specification, and that the vacuum pipes to the CPR operate correctly from the thermo-vac switch. (Assuming that the cold start valve is not permanently operating. Try removing the blue plug from this cold start valve)

[David T]

It will run (and smell) rich when cold and till it gets into "Closed Loop". You will know it is in closed loop because you will see the dwell meter start swinging. In open loop the dwell is a steady reading. Once the dwell starts swinging (easy to see on an old, analog dwell meter) set it to between 35 and 45. If it doesn't "swing" the WOT switch could be stuck, the O2 sensor may be bad or disconnected, or the frequency valve is not working. Before trying to adjust the mixture screw you must make sure EVERYTHING, including fuel pressure, valve adjustments, injectors, spark plugs, all of it, is set perfectly. You cannot "adjust" the motor to run right if something is not right. If the tamper proof plug is still in the mixture unit you are better off just leaving that adjustment alone. It is VERY sensitive. A tiny turn can throw it way off and it is probably closer to where it should be than if you try adjusting it.

[NckT]

It will run (and smell) rich when cold. .....

It shouldn't do if set correctly.

[David T]

The catalytic convertor and the O2 sensor are not effective till they warm up and til they warm up you will get more smells. That being said you should not run the motor in a garage where the exhaust can accumulate. Run it outside and it should be less noticeable. If it is bad maybe you have exhaust leaks and the exhaust is getting into the cabin?

[NckT]

.......

I can't seem to shake the feeling it's still running rich, mainly from the smell. I don't expect the car to smell like a 2017 Accord, but the richness smell is intense. If I do a 5 minute test routine, my clothes will smell for the rest of the day. If this is par for the course, then no problem. Should I aim to tune it leaner even if my readings seem to look ideal? Thanks for the advice!

..........

David, please stop quoting the manual of operation with respect to the dwell cycle etc and read what the original Op has said what is wrong. This very rich smelling condition is due to incorrect fuel pressures or very less likely mixture adjustment, especially considering that the amount of unburnt fuel coming out of the exhaust is enough to make the clothes smell. Item 4 description of symptoms is clearly when the lambda switch open circuits and the fuel ecu references the oxygen sensor and the wild swings described is the result of the o2 feedback to lean out the over fuelling condition. This is why I advised to check the fuel pressures first.

I'm not into the childish immature 'last post wins' mentality either so I'll only respond to the rjd2's original problem.

[FABombjoy]

If cold dwell == warm dwell - especially with no fluctuation when warm - the WOT switch could also be stuck down. Or the thermistor is shorted, wire pinched, etc.

You could quickly test for this permanent enrichment condition using a multimeter to check resistance of the WOT switch to ground. The light green wire should read continuity to ground when cold (coolant is 59F or lower), but should not read continuity to ground when warm.

[David T]

From reading his first post it sounds like he is in open loop. He needs to figure out why it doesn't get into closed loop. It could be fuel pressure, it could be a lot of things. I gave him some suggestions to start looking. I didn't suggest checking fuel pressure because he had no other symptoms to suggest that.

[MikeWard]

Running in open loop does not cause strong fuel smell.

I agree with Nick, fuel pressures should be checked to confirm the CPR and PPR are functioning correctly.

I had a similar issue on my car which turned out to be my CPR was out of spec, and caused a strong smell of fuel which burnt your eyes if you were anywhere near it. Nick very kindly recalibrated the CPR for me and that resolved my issue.