MFI pump adjustments for cold weather

[porsche gt3]

Hi,

I have a 2.8 twin plug MFI engine that runs awesome above 60 degrees, less so between 50- 60 and terrible below 50 degrees.

Its my understanding that the pump needs to be adjusted richer in colder weather.

It took a long time to get the motor to run this well and I am hesitant to tinker with it in fear of messing it up.


What do other MFI owners do if they want to drive their cars in the colder weather?

Also, is anyone familiar with a product that can supplement the MFI pump by adding additional fuel into each bank during colder weather that works electronically and has a sensor that measures ambient temperature?

Thanks

[manbridge 74]

What is your CO or AFR at idle and part load?

Mine is 2.7RS, very close to way it came from the factory. Runs great from 20F to 95F. But I am operating mainly from 6500-8500 feet. I’ve also set the part load and idle with a wideband O2 sensor under load slowly creeping up to but not right at factory CO specs. My plugs are W3DPO year round.

[RSTarga]

I drive mine from freezing to 100F, runs great at all temps. The thermostat should take care of the mixture variations for different temps, just as the barometric regulator adjusts for altitude.
Is the problem the same at the beginning of a drive as when the car has reached operating temperature? How long a drive are you talking about.
Factory CO specs don't work anymore with our new fuel formulations.
Have you done a CMA run-through?

[RSTarga]

Are you sure you have the proper space cam for a 2.8?

[porsche gt3]

Thanks for the quick response.

I have not had the CO or AFR checked and after a half hour ride in cold weather, the motor still hesitates and is looking for fuel except under full throttle.

I have been relying on my mechanic to get it to run properly and was told this is the nature of MFI.

The 2.8 is not factory spec, but instead has smaller cams and a modified space cam per the builders spec.It also has smaller valves, intake and exhaust ports then a true 2.8. I am not sure of exact sizes without checking the build specs.

I drive the car on the street and the car pulls strong from 1000-7400 rpms (above 60 degrees) A true factory 2.8 would not be very street friendly.

I am wondering if something is missing or not connected that helps the motor compensate for cold weather fuel needs. It stinks that i cant drive the car in cooler weather.

[mikael_s]

Do you have the MFI thermostat stack? (I don't think 2.8 RSR motors did)
In case you don't, maybe adding a set screw in the thermostat blanking plate so you can manually richen the mixture at colder temps would work.

[ed mayo]

Do you have headers or heat exchangers, if heat exchangers do you have the heat tube from the fan housing to the heat exchanger (probably not since the distributor is so big), do you have the hot air tube from the heat exchanger to the pump thermostat ? If the answer is headers then you have none of this and that is your problem.

[porsche gt3]

I have to look at the pump and check.

Is the the thermostat supposed to adjust the fuel mixture based on ambient temperature? if so, this is probably the culprit.

Would the thermostat deprive the motor of maximum performance and is this why it is not used on the RSR motor? I assume the RSR gets used during race season and only in warm weather, hence no need for thermostat.

[porsche gt3]

I have SSI's with heat. I am actually looking to get rid of the SSI's, heat and seal the vents that carry air to the cabin and then install headers. The motor runs rich with lots of cam overlap and when the heat is on it stinks up the cabin.

[mikael_s]

I think the thermostat only enrichens the mixture until the engine is up to operating temp.

There are no heat exchangers on the RSR to get the hot air to the thermostat from.

[ed mayo]

The thermostat continues to operate as long as the engine is running and keeping the thermostat hot. That is what leans the mixture to the proper hot A/F ratio. If you have no thermostat the mixture stays at whatever it was manually set for.

[porsche gt3]

Thank you, very helpful
If I install a thermostat if missing or fix the one on the car, will it work if I install headers in the car with no heat?

[mikael_s]

Ed, at what temp is the thermostat stack fully extended?

I was under the impression it only provided enrichment to a cold engine and stopped enrichening at a pretty low temp. So that once the engine is warm (be it in hot or cold ambient temps) it no longer provides any enrichment.

It's gotta be pretty low ambient temps if the engine can't reach operating temps.

[ed mayo]

In order for the thermostat to get a supply of hot air it has to come from the heat exchanger, and in order for the heat exchanger to force hot air to the thermostat it has to have the heater tube from the fan housing to the heat exchanger, so that the engine cooling fan pressurizes the heater tube, which pressurizes the heat exchanger, which pressurizes the hot air tube to the thermostat. Engines with headers and no thermostat have the pump set to a desired setting at a specific temperature, which is fine as long as you're in that range,,,,,,which is where all this started!

[mikael_s]

Gt3: no, you need hot air feed to the thermostat stack. The MFI heat exchangers have an outlet for that. Not possible with headers withoutheat exchangers.

[ed mayo]

Generally once the thermostat housing reaches about 100 degrees F it is done leaning. If a running complaint on a customer car the first thing I do is try to lay my hand on the thermostat housing, if I can leave my hand on it then it's not getting hot enough.

[porsche gt3]

Thanks guys. Now that you figured out my problem, what options do I have to make the car run better when it’s cold out? Is the manual fuel adjuster you mentioned before, that bolts to the thermostat blank a good idea, and how hard will it be for me to adjust? Any electronic fuel enrichment devices available to supplement the fuel deficiency during cold weather?

[mikael_s]

First thing to figure out is whether you have the MFI thermostat stack installed or not. If you do you should make sure it works properly.

Do you have an A/F meter installed?

Look up the CMA manual (Check Measure Adjust), it describes how to approach MFI.

[tobluforu]

Take a few pics of engine.

[ed mayo]

Henry Shmidt has a cable operation to move the rack for cold start enrichment. You could adjust for cold running that way.