Manifod pressure sensor question

[CaptDon]

My 1.7 liter D-jet car was running so rich it would barely keep running. An ultra rich mixture has been a recurring occasional problem. The car also backfired and died on lap 1 of a practice session last April. After being towed to the pits I chacked all connections and found everything ok. It restarted it and other than the occasional rich mixture has had no problem until last weekend.

This last weekend, during Fridays practice sessions the car ran had a flat spot about 3800 to 4000 RPM and an over 5000 RPM miss. If I still had points I would have guessed the high RPM miss to be point bouonce. I went through the ignition system Saturday; replacing plugs (indicated a very rich mixture), cap, rotor and wires (points have been converted to electronic sensor) and insured timing was set correctly.

On Sunday, the car ran great coming off the trailer. It ran progressively worse during the morning practice session not wanting to accelerate unless I pumped the accelerator. If I held it solid against the floor the car would bog down. After the practice session I rechecked everything in the ignition system and found all ok. I went out for qualifying and the car was missing so badly I couldn't get any speed up. Upon returning to the pits after 1 lap, the car was belching black smoke and missing.

I pulled the MPS today and tested it according to Kjell Nelin's article; across terminals 7 and 15 with my digital multimeter, and got a reading of 0 ohms. (should be about 90) I then tested across terminals 8 and 10 and also got a reading of 0 ohms. (should be about 350) My question, did I do something wrong in the testing? would the car even start and run if there was no resistance across both sets of terminals? What advice do any of you members have to resolve this problem? I also suspected a vacuum leak but could find no such leak while at the track. Is it normal for a MPS to intermittently fail before failing completely?

Thanks in advance for any and all help. Don

[Dave at Pelican Parts]

I'm guessing that you tested the MPS incorrectly somehow. I'm not sure how you could, but having both sets of windings read zero ohms (not infinite, right?) would seem to be difficult to do.

Use a hand vacuum pump to see if the MPS holds a vacuum. Mity-Vac is your friend, here.

Check the head temp sensor and the connections thereto. If this is bad you can get very rich mixtures...

Have you checked Brad Anders D-jet site yet? http://members.rennlist.com/pbanders .

--DD

[pbanders]

I'm not sure how, but you probably were testing the MPS incorrectly (e.g. problem with your meter?). If you had two shunted coils in it, you would get zero injection pulses.

As DD says, very rich running is one of the following (or more) things:

1. Leaky MPS (won't hold a vacuum)
2. Open CHT sensor
3. High fuel pressure
4. Leaky injectors
5. Leaky cold start valve

Check my web page for more info.

[Jake Seidel]

I Think it is probally the cylinder head sensor. I changed my points,caps ect and my car 1972 1.7 is running really rich. I looked and found the sensor not even pluged in. I have a problem where it plugs into and how.
thanks
Jake

[Dave at Pelican Parts]

The CHT sensor is not plugged in? That will cause a very rich mixture...

Look for a plastic box connector on one wire sticking out of the fuel injection wiring harness near the right-side injectors.

--DD

[CaptDon]

Thanks for the comments guys.

I did a little preventative maintenance on my multimeter (replaced the battery) and now get readings across the coils. I get 91 ohms across the first set and 341 across the other terminals. I just purchased a vacuum tester yesterday afternoon and will check the diaphragm as soon as time permits. I'll also check the CHT.

I really didn't think the car would have run with the erroneous readings I got the first time.

Once again thanks, Don

[Dave at Pelican Parts]

The CHT sensor is the main suspect. Especially if it isn't even plugged in!!! If it hasn't been plugged in for any of this, my question is why the heck did the thing run OK for any time at all???

--DD

[scotty914]

just for giggles check the fuel return line if it is clogged your fuel pressure will be ok for a while then it will go up. mine did this and it drove me nuts for a month of testing and cleaning.

my car would run great if it sat for 20 mins and fine at highway speed but in traffic it would bog sputter and stall, i had to keep reving it to keep the fuel pressure down ( or i discovered this )