Unusual MFI Problem

[Jeff Higgins]

I have never seen this before. This was happening with one of my good buddy's cars, motivated by his X-Factory built 2.5, twin plugged short stroke. You know how a carbureted motor will bog if the throttle is opened very rapidly if the accelerator pumps are not squirting? They make that kind of hollow gasp, won't rev, and will even die altogether if you get too aggressive on the throttle? But if the throttle is opened more slowly, it revs just fine, with no indication of trouble. Well, I have never seen MFI behave that way, until this time.

We put our heads together and tried to come up with what might have been happening. It was as if the stylus was not tracking on the space cam. Like its linkage or something was gummed up. Slowly rotating the space cam when the throttles open, the stylus seemed to be tracking it just fine. Moving the space cam rapidly acted as if the stylus was left hovering above it for a split second, not keeping up, not providing enough fuel to let it rev.

What would cause such a thing? We eventually reasoned it had to be in the "fuel head" area, where the pistons, their sleeves, and the toothed rack reside. We thought it might be varnished up to the point that rack movement was hindered. So, on a hope and a prayer, he dumped a can of Seafoam into the gas tank and went for a drive. About 120 miles down the road, it cleared up. Runs like a champ now.

Anyone ever seen this before?

[rowingone]

" We thought it might be varnished up to the point that rack movement was hindered " I think you answered your own question. Why did this happen . . old gas? . I would drain the tank and replace the canister filter. This wouldn't happen to a car that is run frequently and has fresh gas. If the pistons in the MFI pump are sticking it won't pump enough fuel to keep up.

[Jeff Higgins]

That's the question of the day - how would this happen?

The car has a brand new 100 liter tank, brand new fuel filter, all new fuel lines. And believe me, this car doesn't sit around much. The pump, however, has never been rebuilt.

I don't think the pistons were sticking. Like I said, when the throttle was opened slowly, it ran great. The springs driving the pistons, kind of like mini valve springs, are pretty darn stiff. I really think the rotating sleeves and the rack that moves them were sticking. Just a really weird thing I have never seen before.

[rowingone]

Call Mark Jung at MFI Werks and he will have an answer.

[Jonny042]

I recall a thread with similar symptoms. I believe you are right about the stylus hovering. Recall that the decel cutoff solenoid lifts the stylus from the cam and cuts all fuel delivery, and it's the cumulative spring tension of the 6 clock springs (not the plunger springs) on the plungers that return the rack to normal fueling position. Same is true if you suddenly floor it, you are relying on the clock springs to make the stylus track the space cam. These springs are very, very, very, slight and not exactly wound up to a high tension.

It might be too late now but you could remove a side cover and observe the pinion movement as you manually move them then let them go. They should return convincingly, if not exactly snap back into position.

[Jonny042]

At about the two minute mark I show a "gummy" pump that has poor return action:

[Jeff Higgins]

Thanks for the sanity check, Jonny. This did seem like the most likely scenario, given the symptoms. I think that was reaffirmed by the "cure".

It is a little surprising just how little spring pressure is making that stylus track the space cam. I guess we really don't want it digging a groove in it or anything, and it does work as designed pretty much darn near all the time. The notion that it wasn't was something I had just never seen before. It's reassuring to hear from you, with your experience with this system, and that you agree this is entirely possible.

Oh, and this pump no longer has the decell solenoid. I helped a little bit when X-Factory Al and the owner installed the motor in the car, then I was asked to help tune the MFI. I suggest to anyone that I'm helping out that this solenoid be removed. It really doesn't do anything useful, and can be a source of trouble if it or the rpm sensor go on the fritz. Maybe keep it on a concours car, but for one that's actually driven, it's an added complication with no real benefit.

[RSBob]

“Space (cam), the final frontier”. Who knew?