Hesitant acceleration... MFI?

[gdee123]

I have been suffering poor transition performance for a while: when the engine is up to temperature ( 80C +) the car judders and hesitates on acceleration from low revs. and is fine above 2000rpm. Assuming MFI mixture problems, (CMA preliminaries have been checked out and are OK: Air filter ok, compression ok, injectors not checked, but performance 2500 rpm + is fine; throttles synchronised, plugs, timing, dwell OK) I have progressively adjusted both idle and main racks to no avail. Increasing the 114mm control rod by 1mm improved the situation, but still lumpy. Before a long run last week ( 2X 60 miles) I lengthened the control rod to 116.5mm ( I know I'm not supposed to!). The car ran perfectly with smooth progressive transition from low revs in every gear. After the run I checked plugs and exhaust condition, both fine: plugs a little sooty, but ok around the gap; exhaust clean and normal smell.

However, the running temperature increased to 100C ( = 212 F) and stayed there throughout the runs, despite speed etc. This is 15C higher than in used to be, and is on the high side for an unstretched 2.4L, I would have thought.

One anomaly is that the MFI pump is not correct, its .... 014 Bosch no. and is correct for the '73 911E, it was exchanged years ago.

My questions

1. Why the temperature increase, should I be worried
2. Do any of you have a view on poor transition performance with MFI.

[cmcfaul]

212 F is exactly what you want the car to run at. This engine temp will give the longest engine life for that car.

Double check the timing and enjoy it, sounds like you have the MFI dialed in.

Chris
73 911 E

[Chris Tallon]

You should test the injectors as well. If they are not atomizing the fuel/air properly, you will feel the car underperforming in the low rev range.

[Grady Clay]

Garrett,

About a year ago I did an analysis of a 2.4E pump on a 2.4T engine (unpublished). I convinced myself of the inappropriateness of this. There are BIG DEAL mixture issues through the rpm and throttle position ranges.

I wouldn’t even begin to offer suggestions to solve your running issues without having the system internally compatible.

That said, your symptoms are lean running. Sure, you can richen the system and have the lean problem go away. All that does is cause too-rich running in other rpm-throttle areas and may cause serious oil dilution possibilities during warm-up. MFI loves (in terms of hiding running issues) to run rich. On the other hand, rich is bad for performance, emissions, fuel economy and (in cold or extreme circumstances) the potentially disastrous oil dilution where the bearings and cams don’t get proper lubrication.

If you adjust everything for proper running at idle, transition to about 3000, and acceptable running at 3500 part throttle (all this may not be possible); you will run dangerously lean at torque peak to hp peak with full throttle.

I recommend you get a properly adjusted 2.4T MFI pump. With everything else correct, you should have a great performing engine. The 911E pump used on a 911T hurts performance and causes all sorts of tuning issues.

BTW, I don’t consider the rpm range from idle to about 3000 “normal” driving. Sure you have to go through that range to get the 911 going. I tend to almost always drive (normal traffic) in the 3000 to 4500 range with bursts to 6000 at any opportunity. That is what a 911 is all about.

Best,
Grady

[WM711]

Hey Grady-I have read many quotable quotes from this site, your last paragraph is Gospel. I hope to buy you a beer someday. Cheers- Bill

[edmayo]

Everything Grady said is true, except I think he meant that you need an "S" pump since you list your car as S not T. There is a reason Porsche had three different pumps, they are not interchangeable. Don't know how you got that E pump, but I'm afraid you're flogging the proverbial dead horse. (silly saying we colonials use!)

[gdee123]

I was afraid that this would be the response. I only noticed the anomaly a couple of weeks ago, it didn't happen on my watch. Grady, I'm completely with you on the fundamental reason we are all in contact here: its just I live in an ( elderly) inner city, and its murder negotiating traffic and narrow streets, hence being in the transition range a bit.

I will look for a suitable s pump in the UK; in the meantime I propose to get a morning in our local institute of technology on their rolling road, and see what the exhaust composition is over the range.

Does anyone have a test for petrol dilution other than the nose?

[gdee123]

OK, I had it dyno'd this AM at Cork Institute of Technology's outstanding transport engineering facilities, and many thanks to Keith and Damien there.

Looks like the E pump was rebuilt ( s space cam?) for the 2.4L S, all the numbers are spot on. Wish I could put the pdf of the curves ( Lambda, Torque, Power - all in duplicate, temp and baro corrected, calibrated and traceable) up for viewing ( don't know how...), but max torque = 158 lb.ft @ 5530 rpm; Max ISO power 189.4 BHP @ 6721rpm; mean ISO power 130 BHP. Lambda indicated rich below 5000 ( 0.8 - 0.95) but OK above: ~ 0.96. Advised to leave mixture be, just check plugs from time to time.

Sorry for waffling on, but I'm thrilled with the results.

[mrbeverlyhills]

I would like to say that Gradys 2 line tutorial on driving a 911 was the best thing I've read in weeks. 6k RPM at any opportunity...right on.

I have a 73.5 cis car that has a recent engine rebuild closer to 2.7 specs: the crank was replaced, e cams etc but I notice the MFI pump is for a 911t. I am sure the pump has been rebuilt and recalibrated(because I asked) but the car runs extremely rich and burps and farts when at fully operational temperature. 2 weeks ago it gave the most enormous backfire (I ducked) and has burped/farted much less but the rich mixture is there still as bad as ever and fuel economy, never a strongpoint, is terrible...9 or 10 MPG?

At idle the exhaust smells so strongly of unburnt gas that one simply cannot stand behind the car. After a Grady run or two to redline and full stop at a light, the smoke catches up with the standing car and one had best roll up the windows fast before asphyxiation takes place. I know the facory would never foist this on the public.

I have read here in the forums how MFI is the hot setup but also how it is difficult to set up and there are few people who can tune it correctly; does this sound like a pump issue? Is this tuneable if one accepts that the pump was correctly calibrated to match the different engine specs? Does it sound like the pump was not mated to this engine correctly and if so, does the pump need to come out for a rebuild? Lastly, am I right in thinking that there is only one soul here on the left coast that does this:Gus at Pacific?

I have only ever had carbs before on my other cars and frankly, that sounds pretty good right now. Thanks in advance for any help, this forum is a great resource for all of us and Grady, you're a star.

[LeafGreen]

The hesitation problem I had was two or three cylinders were way too rich, and when I tried to adjust for it by changing the rod length, other drivability issues surfaced (but the off idle hesitation went away). My final fix was to get Gus to calibrated the pump.

I'm sure my problem was less complicated since the pump was matched to the engine.