Rattling MFI Thermostat

Jeff Higgins · 04-11-2005, 11:48 AM

I was searching through all the old posts regarding MFI thermostats and found one discussing the allowable end play in the stack of discs. It looks like this was discussed briefly but no one really provided a definitive answer. So here is the question: With the thermostat removed from the pump and cold, should there be any end play or "rattle" in the stack of discs? Mine has significant end play. When looking through one of the ports and flipping it end to end, I can see the whole stack sliding back and forth on the rod. Is this normal? Is there a spec for allowable end play when cold? I kind of get the impression from going through older posts that they should be loose, but not to the point of having any end play. Does any one know for sure?

Paul Thomas · 04-11-2005, 12:16 PM

When the thermostat is installed on the pump, it pushes a small "cam" (for lack of a better word) into position making the whole thing tight against a spring in the termostat housing. If it were on the pump and rattling, that would be a problem because there shouldnt be any play.

If you remove the small cover on top of the pump, by the thermostat, and look in there when you put it back you'll see what i mean.

Paul

304065 · 05:47 AM

Jeff.

It shouldn't be flopping around inside at all. There are 25 pairs of discs, two washers, and the circlip on the rod. These are all tightly compressed by the spring on the end cap. WHEN THE ENGINE HEATS UP THE ROD PUSHES OUT, leaning the mixture.

Carefully, and you can lose an eye doing this, remove the two screws that hold on the end cap, and inventory what's inside. Others have claimed to "tune" the thermostat with different combinations, this is uncharted territory and could really screw up your CMA routine.

When exactly assembled, the thermostat is like a light switch in its action. The car gets to a certain temp and then the idle drops. If you shut the car off for only a few seconds and the discs cool, on restart the idle will be back up to 1500, and then drop down to 900 a few seconds later. Very very cool.

304065 · 04-11-2005, 12:37 PM

Jeff Higgins · 04-11-2005, 01:59 PM

Paul, when I pull the thermostat off the back of the pump, it is under spring tension. As I loosen the upper and lower mounting screws it's easy to tell they are under tension. When re-installing, the rod definitely presses the little compensating lever forward, or compresses the spring in the thermostat as I cinch down the mounting screws. I didn't think to check for end play in the stack when it is thus mounted on the pump. I'll check and see if there is any.

John, you are confusing me a bit. I thought forward, in "car orientation", was full lean on that little compensating lever that the "support shaft" in the diagram pushes on. When I take a hair dryer and heat up the thermostat the support shaft comes further out; it doesn't pull it in. I fired up the car once with the thermostat removed just to try to understand better how it was suposed to work. This would mean the compensating lever was all the way back, towards the back of the car. It was so rich it would barely run, belching big black clouds of smoke. So I think back is rich and forward is lean. In the diagram, the snap ring holds the thermal discs onto the fat part of the shaft. As they heat up and expand, they push on this snap ring, forcing the shaft further out. I think. Maybe I'm looking at this all wrong.

304065 · 04-11-2005, 04:26 PM

Jeff, you're right: I had it backwards. Chalk it up to exuberant joy at having conquered (for now) my MFI system. In any case, you still need all the discs. How many are in there? Could you post a pic?

Jeff Higgins · 04-12-2005, 05:58 AM

I checked last night and all 25 pairs of discs are in there. Also, when the thermostat is actually mounted to the pump, there is no end play in the stack. I can wiggle them around with the end of a screwdriver, but they won't slide back and forth. This is when they are cold. Anyway, I've been chasing an intermittent problem where the car sputters, lurches, and belches big black clouds of unburnt exhaust. I thought maybe the thermostat was a culprit, but it looks like everything is normal. It's weird; I can drive the car for weeks and never experience this, and then one day it will do it again. I've been all the way through CMA and everything is in order. When it runs normally, which is 99% of the time, it has never run better. Then all of a sudden it will cough and puke and belch black smoke. Sometimes it clears up right away, sometimes it persists until I shut it off and it sits for awhile. I thought maybe the thermostat was not fully expanding, or the discs were binding, or something like that. I guess I will have to look elsewhere.

Grady Clay · 04-12-2005, 08:07 AM

Jeff,

Here is a thread on the thermostat.
MFI thermostat spacers

From your symptoms it could be the rack occasionally sticking in the rich position. I suppose there also could be an electrical malfunction in the cold start circuit giving a shot of cold start fuel.

That is an unusual problem. The most difficult aspect is it is so intermittent. Are there any other symptoms? What were the results of CMA?

Best,
Grady

Zeke · 04-12-2005, 08:21 AM

Quote:

Originally posted by Grady Clay
Jeff,

]

From your symptoms it could be the rack occasionally sticking in the rich position. I suppose there also could be an electrical malfunction in the cold start circuit giving a shot of cold start fuel.

That is an unusual problem. The most difficult aspect is it is so intermittent. Are there any other symptoms? What were the results of CMA?

Best,
Grady

I'm not an MFI expert, but I've poked around enough to 2nd that thought: Something electrical. Clean all faston wire connections, check wires for continuity while wiggling them and clean grounding connections.

BTW, good way to check just the exposed length of a wire (the most likely area of a break in continuity) is too poke it with a safety pin, or push pin, and connect one end of your meter to that and the other end to the connector at the wire's end (removed from the connection, of course). In the case of a female connection, insert a bare male connection for easy testing.

Paul Thomas · 04-12-2005, 08:35 AM

I had so many problems with my thermo-time switch on hot starts, i ended up putting a button on my dash to activate it when needed. Electrical problems are sneeky, one time my oil pressure was really high and on a whim i clipped the connector off and crimped on a new one, no more problem.

Have you pulled the rubber cap off of the back of the pump and checked the rack movement? If it gets gummed up, it could stick briefly.

Paul

Jeff Higgins · 04-12-2005, 09:32 AM

I did suspect the thermo-time switch earlier. I started a post that many of you chimed in on with some very helpfull information. I have even driven the car with the hot lead to the solenoid at the top of the filter console disconnected so it could not shoot fuel into the stacks even if the switch malfunctioned. It still ran in this apparently over-rich condition with that lead disconnected, so I think I have eliminated the thermo-time switch as a culprit. In running through the CMA, I was able to perfectly synchronize airflow through all stacks, and the mixture checked out spot on when run on a dyno at a local shop. The decell shutoff circut works as specified, and every other check in CMA appears to be o.k. I even suspected the Pertronix Ignitor I had installed about a year ago was going bad, so I went back to points. No change. I thought maybe the CDI box, but on further reflection, it is very much throttle position related. In other words, if I lift off the gas and let revs drop, it clears up and the car never dies like it would if ignition were cutting out. Kind of perplexing. I didn't realize the rack inside the pump can stick. It was rebuilt at Pacific about 30,000 miles back, so I really doubt it is internal to the pump. One of the other symptoms is that sometimes the gas milage will get noticably lower even if it runs well through a whole tank. That leads me to believe it has still gone on the rich side, just not enough to show up as poor running and black smoke. I do not smell gas in the oil, so it isn't that rich. I'm kind of stumped. I just wish it would keep it up all the time so I can find it.

304065 · 04-12-2005, 04:37 PM

Jeff:

The car is speaking to you. It is challenging you to escalate, to raise the stakes. You must fall back and strike at its heart with modern technology.

It is time for an LM-1 data logger!

LM-1 Installation in Early 911- MFI

Also, have you checked the ground strap? I actually went to the point of installing a separate one between the chain cover and the engine mount to be ensured of good ground for the ignition. A bad ground can cause all kinds of black smoke- and then when the ground strap flops around, the problem goes away.

Jeff Higgins · 04-13-2005, 06:43 AM

I remember seeing your post on the data logger when you first put it up. Cool stuff. For now, it will have to fall into that "some day" catagory on the ol' wish list. And you are right, the little feller certainly is talking to me. Maybe more like teasing me. I'm ready to start looking far and wide at things like the ground strap. Anyway, last night I pulled the rod and discs out again. I noticed it felt like the rod end might be a little tight sliding out of the hole in the pump body. I went ahead and polished it down a bit until it slid in and out much more easily. About five minutes from cold start-up on the test drive, it sputtered a bit and cleared up. I stopped for gas about five minutes after that, and on the now luke-warm re-start, it sputtered oh so briefly and cleared up. I went for about an hour and a half's drive and it ran like a champ the whole time. If it continues to do this, where it might sputter for just a bit after a cold or luke-warm start, I could live with it. It seems like there might be a point in the thermostat warm-up where it might not precisely match the mixture to engine requirements. Is that normal? Right now, it runs great stone cold or completely warmed up.

Grady Clay · 04-13-2005, 07:14 AM

Jeff,

Thinking about what John suggested; if the ignition cuts out, for whatever reason, for 1 sec @ 3600 RPM that is 60 revolutions of unburned fuel in the exhaust. It would follow your description of black smoke.

It might be worthwhile to investigate the entire ignition system – CD box, wiring, power, grounds, transformer, etc.

I think you are correct about the mixture. It is very rich on start-up, goes slightly lean for the conditions during warm-up, and has proper mixture at operating temperatures. This is ideal, slightly lean during warm-up prevents oil dilution with gasoline.

Best,
Grady

304065 · 04-13-2005, 03:06 PM

There's also a ground strap that goes to the back of the electrical console to ground the CDI. The console tends to loosen up and flop around due to vibration. I would check that, too, and verify that the CDI fins are grounded to the male faston tab.