early 911 MFI warmup behavior?

[jkeyzer]

I'd like to know how a properly set up MFI system behaves when you first start to drive a cold engine. Mine takes maybe 20 minutes to warm up and for the flat spots and real low idle (occasional POP out the exhaust) to go away. The hand throttle seems to work somewhat but it can't seem to keep the idle up until I've driven the car at least 10-15 minutes. Should I expect MFI to be somewhat balky for the first 15-20 minutes of driving, or can I do better?

[Matt Holcomb]

Jeff,

It sounds like you have a pump that's in need of a strip and clean. At least, mine does everything yours does (except the popping) during the warm-up cycle, and it needs to spend a few weeks at Pacific Fuel Injection.


Matt Holcomb
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[Wayne 962]

Do you have the warmup hose properly connected to the heat exchanger? This is necessary for proper operation all the time (not just during warmup)...

-Wayne

[jluetjen]

I agree. Yours is not running right. For comparison, after start-up, my car idles comfortably between 1500 and 2000 RPM and after about a mile or two the hand throttle is completely off and it idles at less then 1000 RPM.

[pwd72s]

I'm with Wayne on this one. I once had a warm-up hose break. My car exhibited the symptoms described. Once the hose was replaced, the car ran smoothly.

[silverc4s]

Are you having this problem in a cold climate?
My MFI experience is all warm-weather, but my freshly rebuilt and fully calibrated system is almost as smooth as EFI after about 1 to 2 minutes of warm up only. <500 miles on new engine. I have the priming system disconnected, BTW, making it really hard to start....

[john walker's workshop]

generally, a well tuned one starts right up and with the aid of the hand throttle for 5 minutes, runs quite well. your hand throttle seems like it needs to be adjusted to raise the idle speed more. the adjustment is made thru a hole in the tunnel to the left of the e/brake assembly. loosen the slot screw in the aluminum knob and move it closer to the plastic actuator. loosen it just enough to move it with the screwdriver still in the screw. if it falls into the tunnel, you're screwed.

[jkeyzer]

I will doublecheck the warmup hose. I think that is all working properly. I would be more suspect of an incorrect mixture setting on the pump.

I think the hand throttle will raise the RPMs to almost 3k when the engine warms up. The problem I have is that when I am warming up the engine the hand throttle can only keep the RPMs up for a couple seconds and then they tend to fall back down again unless I give it a burst of gas. My gut feeling is that the correlation of the throttle plates is off or a plug is fouling and causing this.

I was just hoping to hear how a properly working system behaves, so I know what to shoot for.

Thanks!

[john walker's workshop]

you're probably running way too rich, or way too lean. either way, it runs wierd. 1st thing, you need to have a shop check the CO when it's at normal operating temperature at idle and at 3000 rpm. 6-8% in both ranges is in the ballpark and can be fine tuned from there.

[Mike M]

Mine is hard to startcold, but when it does start I can control the idle speed with the hand throttle. It seems to warm up quickly, eventually the hand throttle is all the way down and it idles about 1000rpm. Sounds like your hand throttle needs adjustment.

[Swank911]

Does your Pump still have the thermostat? This may need cleaning. Mine ran rough cold until I disassembled the thermostat.

My hand throttle will bring RPM past 3k when cold. I set it to idle high, after a few minutes the RPMs climb as the engine heats up.

It runs only slightly rough to the end of the block. Then it is all fun

[jluetjen]

I think the hand throttle will raise the RPMs to almost 3k when the engine warms up.

For comparison, when cold my hand throttle could wind the engine up well past 4000 if I'm not careful. Higher still when the engine is warm.

My experience is somewhat "cold climate" based living in New England. The car runs pretty much the same down to freezing, it just needs a little more hand throttle in cold weather.

[jkeyzer]

It seems like it's running too lean until it really warms up. If I push in the pump rod the idle speeds up a bit and smoothes out. I'm going to have to check out the thermostat and make sure it is moving and not hung up despite cleaning it out. I don't remember it doing this so badly before so this may be a new problem.

Thanks for the help.

[Matt Smith]

Jeff- I suspect mixtures too, but that's never really a popular diagnosis here!
Mine used to do as yours. Ever since I dialled it in a couple of years ago, the starting procedure is thus:
Hand throttle up about half way, switch on and wait for the gurgling to stop (1-2 seconds) then spark it at the same time as a jab on the throttle. I've found that if you don't bother to do the latter, there is an annoying possibility of flooding the thing, and only heaps of churning with an open throttle will re-animate.
This usually happens only if I'm lazy (and with a large group of people watching), usually when I restart a short time after the initial start- and I'm assuming is most likely due to the cold start enrichment dumping too much fuel.
But normally (9.8 times out of 10) an instant start and high 1000 to 2000 rpm idle for a few minutes will suffice. It usually starts behaving really well when it's out of the cold zone, but certainly it's very tractable and drivable at least until then.
Keep tuning! Your T will probably like running a little fat anyway: I think it's a trait of our cars- even when new.

[jkeyzer]

Thanks for the encouragement Matt.

Here's where I am at now... The warmup thermostat is working fine - I checked it before leaving for work and it was in the full rich (retracted) state, and once I got there (15 mins later) it was in the full lean (fully extended) position. (This is by peeking through the access panel with two screws on top of the pump.)

The problem I am having now is that if I richen up the idle enough so that it is happy when cold, it winds up too rich when it warms up. (intermittent misfires on one cylinder it sounds like) I'm having a hard time setting the correlation as it seems like some cylinders need more air than others to be "happy", going against my airflow meter. I think that some cylinders are becoming too rich at warm temps and misfiring, while others are too lean at cold temps and misfiring. Keeping all 6 happy at all times is really difficult!!

I am going to check again for air leaks around the throttle bodies when I get a chance. Other than that I'm not sure where to turn next... I would send the throttle bodies in for a rebuild but I hate to have to wait 8 weeks and then find out that wasn't the problem.

You would think after fighting this thing for 9 months I'd have it straightened out.