MFI engine: need to modify pump depending of cold or hot weather?

[SFE]

Questions for people who have a mechanical fuel injection pump in their beautiful 911 !

On my 911E 2.0 liters when the engine is cold I always have to pull the hand throttle a little until it warms up.
Then, once around 80°C, I pushed back the black lever completely and it used to held idle fine around 900 rpm.

Since the arrival of summer, once the engine is warmed up, the idle gradually collapses with some exhaust pops then it stalls if there is no hand throttle.

I tried 2 clicks lean then 2 clicks rich on the idle screw and it was pretty much the same behavior (so back to original setting on the idle screw).
Then I opened 1/4 of turn on the 6 screws on the throttle bodies and it holds the idle but slightly high around 1000 rpm and always some exhaust pops time by time.

So here are my questions:

1/ Do you also have to regularly change the pump settings to have a nominal operation?

2/ What settings could I try to lower the idle to 900 rpm without having the engine stalling?

Thank you !

[edmayo]

I have two mfi cars, once set years ago I don't touch them seasonally. I would look first at ignition. When were pointgap and timing last checked.

[Richy]

I also have two MFI cars . I think the MFI throttle bodies and linkage need occasional adjusting because it is a relatively crude mechanical system , susceptible to wear and somewhat complicated with all the moving parts .
But seasonal adjustments ? probably not .

[dpetterson]

Agree with Ed Mayo here - once an MFI pump is set up properly, they rarely need any kind of adjusting.

Some recent issues I experienced with my MFI car were related to cam timing, ignition timing and something as basic as incorrect point gaps (as Ed mentions above) and did not involve the injection system whatsoever.

Hope you are able to resolve your issues.

[HughH]

I have a MFI car and once I had the pump properly adjusted over a decade ago I have not needed to touch it. At that time I renewed the throttle bodies and had the linkages taken back to exact spec.
The adjustment process took well over a month - with small adjustments (almost all to the internal parts of the pump which needed the side plate removed and oil from the pump drained to make the adjustments) followed by driving in different conditions both with and without an air fuel meter attached to fine tune it before moving to the adjustments that can be made externally to the pump including playing with the hims of the cold start mechanism. However a lot of the complexity was due to the fact that the engine is a 2.7 with an RS space cam and throttle bodies etc but it still has the E cams so it is not a straightforward setting for the pump.

IF all of the things you can adjust (including for ambient temperature) are set properly (instead of one being set wrongly to overcome an issue with another part of the engine or of the pump etc) it should work properly for a very long time without any adjustment. I agree that over a very long period wear and tear might cause adjustments to be needed but I suspect that most of these are due to wear and tear on related parts - not the MFI pump

My experience is that in the past few decades people have adjusted MFI pumps to try to counteract other issues like linkages etc mentioned above, but have ended up putting the pump out of the correct settings and then are constantly chasing their tails trying to fix issues - often by adjusting the wrong thing

[SFE]

Thank you all !

When I bought the car 8 months ago the body and the engine were completely rebuilt.
The only remaining work I had to do was to take care of the injection system and some electrical component/wiring.

So I started by installing NOS throttle bodies and intake pipes, then we checked all rod linkage as per the book.









Then I sent the pump for a rebuild to a specialist.





Then we change the alternator, the VR, the lightning harness, put BP7ES spark plugs. We changed the batteries, the Injection pump relay for the shut off solenoid.
Of course all timing, dwell angle,...were checked by my specialist.

With all this work done it was still very difficult to have the engine working properly, the pump specialist asked us to send him back the pump in order to check his work.
Same result...

Compared to Bosch spec of the MFI pump we had to add 6 clicks on the partial load screw and 6 clicks to the black screws, both on the rich side.

Under load the AfR/richness is OK and the car runs very good.
At idle impossible to have better than around 12 AfR, the idle is inconsistent/fluctuating and if I set the RPM below 1000 it stalls.
Depending of exterior temperature I still have more or less light pops coming from the exhaust pipe.

Everything is brand new or checked on this car and I feel like chasing a Gremlins !

[HughH]

I feel like I might need to leave the comments on this one to Ed given what you have just explained.
However I have heard from many people that when a MFI pump gets completely rebuilt to original spec it is not uncommon once fitted to the car for more adjustments to be done ON THE CAR to account for the idiosyncrasies of the individual engine

I would not be surprised, given this is essentially a totally rebuilt engine from the sounds of it that small problems may exist in different places. For the idle setting I would be looking at the distributor settings etc first - not the MFI pump

However it could be that the thermostat is not working properly and still trying to add more fuel even when the engine is hot....

If the main external adjustments dont work as you suggest (or fix it at one set of revs and make it worse at another...)

Then I would be looking at the racks/ governor INSIDE the MFI pump (only adjustable by taking the side plate off). The first one sets fuel settings up to 2700 rpm I think from memory. Then IF that one needs adjusting (leaning out at low revs) you will probably have to redo the other one that deals with higher revs and while doing that you will have to be mindful of the crossover point at about 2700 revs not to create a hole at the crossover point

[frederik]

You should go through all of the steps in the Check-Measure-Adjust manual (I can send it to you if needed) before adjusting the pump. Seasonal adjustment is definitely not needed.

The thermostat can be tested by blowing hot air with a hair dryer onto the hose... this should affect AFR.

[72targa]

Check-Measure-Adjust

[SFE]

I feel like I might need to leave the comments on this one to Ed given what you have just explained.
However I have heard from many people that when a MFI pump gets completely rebuilt to original spec it is not uncommon once fitted to the car for more adjustments to be done ON THE CAR to account for the idiosyncrasies of the individual engine

I would not be surprised, given this is essentially a totally rebuilt engine from the sounds of it that small problems may exist in different places. For the idle setting I would be looking at the distributor settings etc first - not the MFI pump

However it could be that the thermostat is not working properly and still trying to add more fuel even when the engine is hot....

If the main external adjustments dont work as you suggest (or fix it at one set of revs and make it worse at another...)

Then I would be looking at the racks/ governor INSIDE the MFI pump (only adjustable by taking the side plate off). The first one sets fuel settings up to 2700 rpm I think from memory. Then IF that one needs adjusting (leaning out at low revs) you will probably have to redo the other one that deals with higher revs and while doing that you will have to be mindful of the crossover point at about 2700 revs not to create a hole at the crossover point

Distributor settings are checked (Dwell angle,....).
The thermostat was checked with the overall of the pump but I can check it again it's easy work.

I don't think the problem is coming from the pump so I do not plan to open it again.

The car is running very very good.....except at idle where all parameters are completely unstable, I will post a video.