Pretty rough when cold?!

[areznik]

Hey guys,

My Q '94 117K seems to be just fine (knocking on wood) except for the fact that it is very rough when the engine is cold. Today we had 45F here in Bloomington, IN and after I started the engine it almost felt like one of the injectors was missing (I don't know what's the cause but that's how it felt like). After the engine reaches its normal operating temprature it behaves just fine. It is hard to imagine that this is a normal behavior under cold start. What puzzles me is the buzzing sound from the fuel pump (or the regulator) in the trunk in the first 20 secs. I also noticed after careful listening that there is a sound coming from the fuel pump as if there is a short circuit or something. I replaced the pump about 20 months ago and it was about 15-20K ago mileage wise. The pump does not produce the "hummmming" sound and engine has plenty of power.

Does anyone experience the same kind of behavior at cold starts as I do?

What are your thoughts on this?

Andre.

[Jesda]

Did you ohm your injectors while cold and while hot?

http://www.q45.org/ohminjectors.html

[areznik]

Yeap, I ohmed them when I did the plenum job last week. All of them were slightly above 10 ohm (i.e. 10.1 - 10.4) and the engine was obviously cold. Out of curiosity, is the resistance of the injectors should change as the temprature of the engine changes?

[DAEDALUS]

Not sure by how much, but definitely all metals change properties when temperature changes, including resistance. Injectors have a single copper wire coiled about in hundreds of loops. Probably most of the difference you're likely to see is from contact resistance changing with temperature.Flooding/too rich would be my guess on the roughness. You used a factory pump for replacement?

[areznik]

As far as I remember it was not an OEM pump. With this, the fuel pressure is 34 psi at idle and goes up to 43 psi at WOT, which indicates that the pump does it job as it is supposed to. Do you think that maybe it takes some time to heat the cats ... . I noticed that during a couple of minutes after the start up there is no smoke from the exhaust as if the engine is shut off. This happens only when the engine is cold. Is this a normal thing?

Andre.

[DAEDALUS]

Sure cats take time to heat up but that's not related to the engine running rough. The "smoke" you're seeing after a couple minutes of running the engine is probably just steam from condensed water in the exhaust. Moisture can't condense in a hot pipe.Perhaps the cold weather is affecting injector seals? When the engine is running rough get under the hood and start wiggling harness connectors--MAF, CAS, injectors you can reach, etc. Again, contact resistance might be the problem. Unfortunate that these older cars are reduced to such barbaric methods, but they are quick and cheap.

[artandscience]

I found I have problems with my MAF when it is cold. Summer time the car runs beautifully, in the winter it definitely seems a bit rougher. Dealership already replaced the ground line to the MAF (common source of problems).

I would suggest that might be the problem. I don't know if it is possible to actually test the MAF without a Consult unit though..

cheers, S.

[DAEDALUS]

You can watch voltages with a DMM. Look for rapid fluctuations that don't correspond to pedal movement.

[Q45tech]

But the rpm must be steady as the MAF changes with flow........hard to decide if 1.21 or 1.23 is right or wrong depends on air temp and pressure humidity. 10% rpm change [650> 715] will change the flow 10% [grams per second]............hopefully you can visualize 1400 rpm >650 is a 50% air flow change so the MAF will change by a ^3.5 factor........use the cube root as a ballpark guesstimate.

Anyway, Bosch type MAF are very inaccurate at low flows [idle] the ecu uses rpm and coolant temp as the main reference......the MAF voltage is just sampled during warm to make sure something is there [close to what ecu expects] +- 50%...............once coolant temp sensor shows 150F the MAF is used almost 100% with rpm.

So from 150F-194F the coolant temp is ignored to speed ecu calculations, a fixed 174F is substituted to stablize minor variations in head temperature.

Study the coolant temp reading vs your problem for clues.

Probably some kind of IAC problem or the hot wax offset screws [fast idle cam setting lift off/ramp] on TB have been fiddled with.