FI problems when car is cold

[RichMason]

I have a 73 914 1.7 with the stock FI setup (except for a K&N cone filter). I have been having problems with my car in the morning when it is cold (20-40 degrees). The car stumbles and bucks and backfires into the intake (as opposed to out of the exhaust) until it gets warm. If I give it full throttle, it will run normally but less than that is a problem. The idle is nice and high until it warms up so I believe that the AAR is working. I also have a new head temp sensor. Now, here is the kicker, I replaced a defective MPS a few weeks ago with a used 2.0 unit. The car was running similarly to how it runs now when it was cold and way rich at idle when it was warm and the vacumn diaphragm was clearly blown. With the 2.0 unit the car performed flawlessly. I put a 1.7 unit on it last week and my problems started again but more seriously. I guess my question relates to how many other things need to be adjusted when the MPS is changed or are there other things that need to be checked first. The car had a full tune up 2000 miles ago with all new parts but I was unable to set the mixture properly at the FI brain because the MPS was defective. I have moved the mixture adjustment around in both directions but it doesn't seem to affect my issues with the cold. Any suggestions?

[pbanders]

It's hard to say without a full diagnostic, BUT...

Backfiring and stuttering when cold sounds like it's too lean to me. When you replaced the head temp sensor, are you sure it's the right one (either a 0 280 130 003 or a 0 280 130 012)? If you accidentally used a 0 280 130 017, the car will be too lean on warm-up.

A "quick and dirty" way of seeing if a lean mixture is the cause of your problems is to unplug the harness connector to the air temperature sensor. This will richen the mixture by about 5 to 10%.

Have you checked your fuel pressure? Verified that you don't have one or more clogged injectors?

I'd also be concerned about the MPS. Either or both may not be adjusted properly or have some other problem. From the limited samples of 043 and 049 units I've characterized, the only difference is in the full-load mixture. The part-load mixture is the same, so switching between the units should have no effect for part-load operation.

The MPS knob only affects the mixture under idle and overrun conditions (car in gear, coasting with the throttle closed). It has no effect on part-load operation.

Brad Anders

[scotty914]

check the cold start injector to see if it works properly

if it is leaking it could cause the stumbing by being very rich but under full throttle the motor is getting much more air so the problem woukd not be as rich

[retro74]

First, Paul and Scott... We have to stop meeting like this.

Scott, thanks again for help with my engine drop, you're a Prince.

Stumbling...
Same problem here on my 2.0L D-Jet and I found quite a few interesting things. It started when my fuel pressure was returned to normal settings. Then I tweaked the MPS screw and it was fine. Then I got a rebuilt MPS and it was not fine.

Turned out to be my distributor. Three main problems with my distributor.

First is the vacuum retard. My vacuum can has a hole inside and it was not pulling the points plate to the retard position at startup or at low rpm. It just sat in the "parked" position.

Second, rust and age caused the 6mm ball bearing inside to decay and rust and groove the plate. This made plate rotation difficult, even if some vacuum pressure was applied.

Third, rust, decay, age, etc. caused the distributor weights to bounce around. I could go on and on about repairing that situation, but it's not a small project as I'll outlinebelow.

I got a rebuilt distributor from PartsAmerica for @ $65.00 and the car was happy again but exhibited a strange flat spot from 3000 to 3300 rpm's.

To paraphrase Paul Anders message on his excellent D-Jet page... "These [D-Jet] recommendations assume that your ignition and tune are in excellent condition."


Everything I learned from Ray Greenwood on TypeIV network...

Start with your unit and quickly move to a rebuilt unit if things are shot. I didn't like my rebuild so I used the parts from it to rebuild my original.

Jack the right rear tire up, put the car in 5th gear and get the distributor rotor pointing to TDC mark. Check the impeller for TDC. Remove the cap and rotor and loosen the 10mm distributor clamp long-nut. Rotate the distributor to the right and remove the 13mm nut from the distributor hold-down clamp and remove the distributor.

Does the drive shaft of the distributor have a spacer between the distributor body and the drive dog (the notched part that matches to the engine)? Is the spacer in good condition? These cannot be replaced with Nylon because of heat. Get a replacement from another unit or make a washer from PEEK or Torlon plastic (about $85.00 per foot). Too much free play here and the shaft is going to move up and down from vibration and cause poor control of the points. This is one reason that optical / Hale sensors can clean-up a bad running car... These sensors can move up and down a bit and still be 100% accurate on rotational timing, while the points movement is mechanical and prone to vibration and other mechanical problems.

Rock the lower part of the shaft back and forth (wedge somehing into the space between the drive dog and shaft to remove the drive dog free play). If you can feel play with your bare hands then it is worn-out. You'll need a dial indicator to test the end-play for real movement, but visible movement is a sign of totally shot bushings. Replacements must come from a doner. Bearings are not available (yet).

If you replace the spacer, the drive dog is off center, so carefully note the drive dog offset against the hole in the upper shaft mounting plate for the weights. Good luck getting the pin out. An Arbor press or Flute pin press is best, but it can be done with a good punch and a board with a U shape cut into it with a 3/16" hole in the center.

Does the vacuum can hold pressure on retard (part that faces in toward the distributor)?

Remove the machine screws from the vacuum can and the distributor plate. Remove the springs from the weight perches.

Remove the trigger contacts from the lower unit. Clean them up with contact cleaner. Oil from maintaining the distributor shaft can reduce the resistance in the contacts and cause poor running until the contacts are shaken clean from running for a few minutes. Be gentle with them.

Note the position of the plate that hold the weights. One side has a hole in it. Note the position of the key used to hold the distributor rotor in place.

Remove the felt from the center of the distributor shat. There is a C shaped clip in there. Take a punch and smack the nose of the internal shaft and the clip will roll over the retaining groove and free the upper shaft. Yes, it comes out that easily!

Clean up EVERYTHING. I use metal etch (from POR-15 people) to pickle the weights and metal pieces and springs. It leaves behind a Zinc coating that keeps the rust away after assembly.

The weights have a small bushing below them. mine were worn out and I used stainless 1/8" washers and a jewelers file to clearance them over the weight pivots. This allows the weights to sit flat (horizontal) to the lower mounting bracket. You can use nylon electronics panel spacers for this, but nylon hates heat and hates oil in the long-run.

At this point, wire wheel the shole thing. It looks great all shiny and smooth.

Reinstall the weights and springs, etc. Bend the outer spring perches until they are not tight and not loose against the springs. This is critical to prevent the weights from bouncing.

Reassamble, put the C clip back in, two drops of oil go down in the distributor center shaft. Clean up the points plate (try not to sand it because it is plated). Replace the 6mm ball with a new one if needed. If the plate has a groove from the ball, file it down with a jeweler's file. It will corrode, so use some grease under the ball.

When you replace the vacuum can, make a paper gasket (with gasket material available from your FLAPS) to create a seal at the can. Water and normal dirt enter the distributor from here and this gasket will prevent future contamination.

Make sure you have a nice solid O-Ring on the shaft and reinstall it. A bit of shaking and pressing. Align the distributor Rotor to the line on the body. Turning the body all the way to the right will allow you to tighten the 13mm retaining nut. Bring the distributor body 2/3 back to the left and you should be able to get it started.

Set the dwell, do the timing thing, etc.

Piece of cake right!

Most of all.. Enjoy

Joe

[RichMason]

I think I am going to be sick...suggestions (although very useful and informative) like that make me appreciate the simplicity of my diesel engines! When do you guys have time to work and families and things like that when you are spending days rebuilding distributors?! Seriously though, thanks for the electrical tips. As I mentionned to Mr. Anders in an email that is not on this posting, I am amazed at the variability that I have experienced just with different MPS and different temperatures and different driving methods. Sometimes my idle drops to zero at every stoplight and other times it is rock steady at a 1000. When I bought my last MPS (used from a parts car), I tested it by sucking on it to hear the diaphragm move and see if it held vacumn and by plugging it in. It was great for the first 10 miles but then things started to change. I think I agree with Paul-Brad, just buy a new one and eliminate at least one parameter from the equation. I think I will go with a new distributor while I'm at it.

[pbanders]

Joe's comments about the distributor are spot-on. Part of my education on how to fix problems with my car came from my mechanic in CA, Bob Grigsby. His approach to diagnosing engine problems brought to him by customers was the following:

1. Listen to the customer's complaint and note it.
2. Go through a fixed diagnostic procedure that covers:
a: basic mechanical condition of the motor
b: ignition
c: fuel system
3. Fix any problems noted (assuming customer has authorized)
4. Check to see if the customer's original complaint is still present. If so, go after it. But in 90+% of the cases, the problem is gone.

You really have to make sure that everything is in order before you start chasing down a FI problem. I keep records of all of the measurable parameters on my engine so that I can see if anything has changed when I detect a problem (compression, idle vacuum, FI component resistances, etc).