So for the past 2 months, I've been trying to figure out why my '73 2.0L with stock EFI is running way lean and hot.

Backstory. I like gauges. I have the Dakota Digital cylinder head temp gauge and oil pressure and temperature gauges. My wife and I took it out to Santa Fe for the Route 66 meet. While we were out there, the MPS diaphragm blew on the interstate. When it blew, it began to run very lean and I didn't let the head temperatures get above 380F. We carefully slowed down rather quickly and pulled off onto the shoulder. Even at idle it was around 360-370F. We shut it off. We never lost oil pressure, but it got very close to the red zone on the oil temp gauge. Fortunately we were out there with @ClayPerrine and @Betty and their trailer.

As per my request, they made this wonderful emoji for me:


In the fall of 2019, the EFI wiring harness was replaced with a brand new one. All of the injectors were sent to Mr. Injector for flow testing and refurbishment.

We also have another '73 2.0L that is a restoration project. It is in lots of parts making it easy to borrow parts from for testing.

Since then here's what we've done:

1. Oil Change
2. Valve adjustment
3. Stole the MPS out of our other '73 2.0L until I repair the first one. (This one holds vacuum and still has the original seal that it has never been opened or adjusted.)
4. Confirmed the correct way the MPS plug goes with a multimeter and the ECU end of the wiring harness.
5. Confirmed timing was set correctly (27 @ 3500 RMP)
6. Confirmed the dwell angle was correct.
7. New points, rotor, cap, condenser.
8. Confirmed all of the injectors were firing.
9. Confirmed no vacuum leaks (and that vacuum lines were correctly routed).
10. Confirmed that all of the blades on the fan were intact.
11. Confirmed the air flaps in the tin were working correctly.
12. Confirmed the resistance of the head temp sender and ballast resistor.
13. Confirmed no blockages in engine tin.
14. Confirmed the MPS and ECU numbers matched and were correct for a '73 2.0L.
15. Confirmed fuel pressure was set correctly (29 PSI) and drove with a gauge to make sure it was consistent and there were no dropouts.
16. Confirmed the ECU was functioning properly with the factory ECU tester.
17. Stole the ECU from the other '73 2.0L, confirmed it was functioning correctly, and tested it it in the car.

The car has been on various test drives throughout this diagnostic process. In all of them, head temps cruising around town at 35-40 mph have been in the 340-350 range. This is about 50F hotter than it used to run. In addition, I have checked all of the spark plugs and they are bright white indicating a very lean mixture.


What am I missing? Clay and I have been trying to figure this out for almost 2 months now. At this point, I don't know what else to check.

You checked the cheater resistor on the cht wire? never mind, i see that in your list

A few things...


1. A torn MPS will cause you to run rich, not lean.

2. If your gas has ethanol it in (which most does these days) a factory tuned MPS will be running lean by default.

3. Cant trust reading plugs with gas formulations these days, get a wideband O2 sensor if you are concerned about the mixture.

4. 340-350 head temps are not really that unusual for a stock cam d-jet car, especially given you are in Texas which is assume is rather warm right now.


I suspect the 300 deg head temps you were seeing before may have been due to 1 cooler weather and/or 2 your MPS may have been failing previously before it gave up completely causing you to run slightly rich which dropped your temps.

In the absence of actual data from a wideband O2 sensor the only other thing that comes to mind to check/clean/replace are your grounds.

Double check the MPS calibration. You need LCR meter and Vac gauge.

https://members.rennlist.com/pbanders/manif...sure_sensor.htm

You said it was stock EFI, but what about the engine. Is it stock displacement? When I first fired up my 2056 with a stock MPS, it ran extremely lean.

Also, I had a similar issue about 35 years ago and it turned out to be a dead cell in the battery. The ECU apparently did not like low voltage. When this happened the exhaust pipes were glowing red hot.

1. I've had 2 MPS diaphragms break. Both times, the car ran super lean and went back to normal when a non broken MPS was used. I know the pbanders D-Jet site says otherwise, but that has been my experience.

2. Yes. The gas has ethanol, and I've always filled up with ethanol gas. I've never had a problem before.

3. I'll be getting a wide-band O2 to double check the mixture.

4. Yes. It is warm right now, but it was just as warm in May/June and even hotter the past 2 summers. Previously, at 35-40 mph, I would have head temps of 290-300 (and this is in the summer months). I was seeing head temperatures of 340-350 at 75 mph on the interstate, even when it was 110 out.). To suddenly jump 50F is a problem. Head temperatures reach 370-380 to go 55-60 mph, and that's just entirely too hot, and it's all interstate to get around Dallas/Fort Worth.

In the winter months, I would have head temps of 270-280 for 35-40 mph and 300-310 for 55-60 mph.

I double checked the grounds last month, but I'll do it again so I can add it to the list of things I've done.

I have referenced that site extensively. I own a LCR meter and have checked the MPS with a vac gauge and meter. Everything checked out when I had been going through my stash of spare MPS last fall. I double checked if the MPS held vacuum when installing it, but did not double check the inductance readings (as they had been fine before). I guess I'll double check that since I'm running out of options.



It is a 2056 and has always run on stock MPS with no issues.

The battery is a newer optima battery bought last year. The voltage on the gauge has always been good (but we know how accurate those are!). I'll double check it with a multi-meter.

If I understand correctly, the MPS went bad and as a result the car ran lean. Replacement MPS gives higher than normal head temps which may be a sign that it’s still running lean.

If the above is correct, then how could it be anything but the MPS?
Do you measure the inductance #s on the first MPS before it blew because that’s what you’d presumably want to use calibrate the replacement MPS.

Since your engine isn’t stock, I would expect a stock MPS to require calibration for AFR.

Doesn’t add up
I’m sure your info is correct and I believe in your work. But it still doesn’t add up? I’ve blown /lost a few MPS And it always goes rich

With the larger disc placement. And modern fuel. No way the stock MPS is going to run the correct mixture.

The wide band is the correct tool. You may want to buy a diaphragm from CFR. You need the tools also from cfr. Put the replacement diaphragm into the blown unit and set the mixture with the AFR

Could I ask what you did to enrichen it? I’m at the fire-up stage on my 2056 rebuild and just about got it idling ok ready for its first road trial, and I’m not sure whether the original D Jet Will need tweaking, not that there’s much to tweak. Sorry for the thread hi-jack.

Thinking a little more.

Maybe clean and check your FI points too

I wonder if the cam blocks are worn if they would open for too short of a pulse?

Yes. The last 2 times the MPS failed (I have 3, 2 need to have the diaphragm replaced), the car ran very lean and this was confirmed with a wideband O2 meter. The MPS was swapped with a known good one with the correct part number, and the car ran correctly (again, confirmed with a wideband O2 meter).

I had measured the resistance of the MPS coils and the inductance calibration at some point a while back when I was organizing parts. I will double check the current MPS to make sure the 2 coils are of the correct resistance and that the inductance calibration is correct. When I installed it, I confirmed that the MPS held vacuum.



See above. Checks have been done in the past with a wideband O2. Confirmed running lean with blown MPS diaphragm and running correctly with the 2 good MPS in the past. I will buy replacement diaphragms from Tangerine Racing to repair the 2 broken MPS and see if the problem goes away when I switch back to one of those.




I will double check those. Thanks. I've got one or 2 spare sets of FI points that I can always swap in and see if that makes a difference.

also...
when you do rebuild one of your MPS bodies...

I think all of the 2.0 MPS (not 100 percent... but I think) have a spacer ring inside that allows the diaphragm a longer range of motion for the 2.0

make sure you either use a 2.0 body that has the ring
or add the ring if you don't have it.
you will be tuning the 3 settings anyways to match your engine (with the AFR)
and the spacer will give you extra fuel/a deeper throw

brant

Update time on the lean issue.

I used an LCR meter to check the resistance of the MPS coils. Both were good. I then checked the calibration, it was also good. (this is a factory sealed MPS for a '73 2.0L).

Meanwhile, I replaced the diaphragm on a second MPS (made sure it had the 2.0 spacer), gave it a rough calibration and tune. Drove it around a bit and got the oil warmed up, then pulled spark plugs. They look great as do the head and oil temps. Now the car is running pretty close to where I want it. Head temps are in the 280-290 for around town driving (in Texas summer!) and 320-330 at interstate driving (70-75mph). I have no idea why the factory MPS which passed every test wouldn't work, but it didn't.

Next step will be a proper tune at some point in the future with a wideband O2. For now, I'm thrilled to have my 914 running again and I'm just going to drive it and enjoy it.

I remember when Ron D. had the 74 bumble creamsickle
he bought a literally still sealed NOS MPS for a bunch of money
had the rivets intact, the plug intact, and tested good. box was still sealed when he got it.

car would not run on it.

I can only assume its some type of original defect in assembly..
in his case, his was the first car it had ever been installed on to.
brant