Help, Car still seems starved for gas when warm

[Eli]

My car has been in shop jail for far too long. My mechanic has finally dialed in the MFI and it is tuned properly. But we are still running into a serious problem. The car starts out very strong, compression is great in all cylinders it runs very well when cold but as the engine warms it starts to act like it is starved of gas and begins to run worse and worse. Since the mfi is properly tuned and the engine has good compression and the timing is correct as well , I'm thinking it must be fuel related.


What would cost the car to run fine when warm and then less and less well within the fuel system?

[edmayo]

What is the air/fuel ratio under load when the engine is running poorly?

[Flieger]

Well, there is a thermostat in there. If it is tuned well when cold, the thermostat would make it too lean warm.

The other thing I could think of is since it's been sitting long the mesh filter screen thing in the fuel tank could be picking up little bits of rust as you run it and causes the fuel flow to decrease gradually.

[HughH]

I will preface this reply by saying I am no expert with MFI unlike some people on the board

with that said I am with Max on the thermostat

I have been having my MFI re dialed in over the past month or so. Over time (the last decade or so) it has been progressively adjusted to take account of little changes in the engine etc
in the past couple of years there have been some significant changes (S throttle bodies instead of E, fixing a few air leaks etc) and the car has been running rich
so we have gone back to scratch - adjusted the main rack, some linkages, the low end and the top end governor (multiple times) and finally the thermostat.

we ended out making significant changes to almost all areas, although some offset others. at one stage it was all fine on the air fuel sniffer at rest but a pig when driving. however when it was almost all right for idle, cold idle and initial cold running and great for acceleration under load across the range, it was still a bit lean and difficult on both transition from cold running to normal temp running and under slight load between 2000 and 3500rpm when running essentially on the governor rather than the main rack.
i took a couple of small shims out of the thermostat (to add a bit of fuel for a bit longer when warming up) and that seems to have cured both.

however when playing around with all of the adjustments i did notice that a change to the thermostat, in isolation, not only changed the cold running characteristics but also had an impact on some of the warm running characteristics - especially at low throttle but with the engine under load.

i would also start with what Ed suggested - work out whether the car is running rich or lean when you get the problem

Then assuming that the cause is it is lean (A/F above 15)
I would try to lean off the thermostat (put shims in) and richen up the main rack or the governors or both. that should add fuel when warm but not when cold. It would also depend on whether the poor running you are experiencing is under load and across the rev range (try main rack first) or at low throttle (governors) and within that over what rev range (over or under 2000rpm)

assuming it is rich when causing the problem i would try the opposite

Also it is a trial and error type of procedure especially when transitioning from cold to hot so it will take some time and patience to dial in on the road rather than on the bench

[Haasman]

How long after starting (cold) does this happen? If you shut the engine down and restart it, will it occur immediately or does it take a few minutes?

Have checked your ignition system when the problem occurs? I have experienced a similar but it was a small hairline crack in the distributor cap. It was small and hard to find. As the engine warmed, so did the cap and its crack expanded. I also recall a friend's engine that would falter after several hours of running, turned out to be the coil.

[Chris E]

I'll throw in a low-tech idea? The classic problem with debris in the gas tank? As the car is running the suction at the pickup in the tank clogs up and starves the fuel supply. As you let the engine rest the suction at the pickup stops and rust flakes/debrisfalls to the bottom again. This would produce similar symptoms to yours and would be fairly consistent.

[Eli]

The problem is consistent, once the car is warm after I would say ten minutes of normal driving the problem occurs. If you let the car sit for just a short time 30 mins to an hour it will still be there as soon as you start driving. It only goes away when the car has completely cooled which takes hours or overnight.

[edmayo]

Sure sounds like the system is going too lean once the thermostat has warmed up, hence you need to know what the air/fuel ratio is when warm and under load, as a starting point.