Remember that arcade game Whack-A-Mole? You take a hammer and whomp the moles as they come up through the random holes? McMark and jcd914 and me had that kind of day playing Whack-A-Mole with the DJet on Jim's engine (jcd914) in my car...

Recently the engine started running bad. It had a low speed miss and then would go like stink when you got into the advance curve. I have a bunch of new 914 DJet components so I started throwing parts at it. This is a method used by people who aren't mechanics to fix something they don't want to pay a mechanic to figure out and fix. Take a note if you aren't a mechanic - this method is often more expensive then just paying the damned mechanic. So, I replaced the MPS with one that I bought from George at AA a few years back and never really used, I replaced the harness with a Bowlsby harness, I put a resistor in the CHT circuit... Each thing I did made it run better in some ways but often worse in others.
So, I decided that since I was having the Sacramento Porsche Family Reunion today and McMark was coming up, how about having him take a look at it and paying him for his time. Mark was game. He brought his LM 1 Wideband AFR and we methodically went through the tuning process. Here is where the Whack-A-Mole game began. Everything we did would improve it but unmask some other issue. Some things did nothing when they logically should have. We replaced the distributor with a rebuilt one from Rich Bontempi, it ran worse. We discovered that two injectors were not firing. So the distributor was good but the trigger points, which were new by the way, were bad. Swapped in the trigger points from the original distributor and it ran on all cylinders again but still ran bad. Lean, very lean... Then Jim Dupree pulls in the driveway and steps out of his car with an MPS that he pulled off of one of his spares. 30 seconds to plug in the vacuum line and the harness plug and the problem is solved. Turns out the new MPS from AA is bad or badly tuned for the motor. I suspect the later as it will hold vacuum.
So after three hours of Mark's time and lots of parts. The engine - Jim's engine runs very nicely again... Ahhh... the joys of a 30+ year old car

If it was easy, we'd probably get bored.

People that want easy cars tend not to like cars older then about 5.

Yeah, it can be frustrating, but damn if it does not feel good when we get it right, eh?

Zach

Good story, and you point out something that I've been harping on recently. Even if everything is right - mechanically, electrically, fuel system, ignition, FI components, hoses, vacuum integrity, etc. - you STILL have to use a CO/AFM meter to verify the idle, part-load, and full-load mixtures, and if they're off, make the appropriate adjustments to the MPS, ECU idle mixture control, or CHT circuit.

I've got two NOS 049 MPS's and two NOS 043 MPS's in my parts store. For each type, the factory adjustment of the full load settings is signficantly different (i.e. 0.2 to 0.3 ms different). I've done some adjustment recently on my MPS and I can tell you that a 0.05 ms change in injection duration for the part-load setting makes a noticable (~0.5%) change in CO. You can't depend on the "factory" settings to get things exactly right.

I'd recommend to anyone who has D-Jet to have at least two, and preferably three MPS's in good condition, with the epoxy carefully removed from the full load stop, and replaced with hot glue gun glue. You'll also need an LCR meter to record your calibrations, and to set the full-load stop position. Take your car to a dyno shop, and using the adjustment info on my page, set your idle (spec), part-load (3.0 to 3.5%), and full-load (5%) mixtures. After you're done, use your LCR meter to record your MPS's calibration curve, and use it later to set up your other MPS's. Re-set your full-load stops with glue gun glue and you should be set for at least 15K miles.

Once you get all of this done, if your warm-up behavior is crappy due to lean mixture during warm-up, get a CHT spacer from Brad Mayeur. I got one, and compared to trying to use ballast resistance with the CHT to fix this problem, it's far more effective. The spacer made a huge difference, my car no longer has the rough idle and surging it previously did, and when the motor is fully warmed-up, it's like the spacer isn't even there. I didn't even have to adjust my setup after I'd installed it.

Hmmm, once I get the new 2.0 in the car, I may need to plan trip to see McMark for some tweaking to the Djet, just to get it all dialed in right. I love these threads, you keep learning new things.

Took you 3 hours, probably saved you hundreds of dollars. Plus you learned something.. priceless. I played "Whack the MOle" thats how I learned the Djet. Still no matter what you do It is hard to suspect new parts especially when that creates 2 suspects.. I took my 914 into the mechanic w/ suspect MPS but driveability was close so damned play with evreything thing for hours. Mechainc found MPS issue as sole problem, luckily he had a new one ( i am probably his only 914 customer so I get what he has in stock rather cheaply, he does not want to have it in his shop anywmore years) total bill about $500.00 You did good..

PBanders, You have a Parts Store? I would like to see it or hear about it.. Or you mean your personal private parts Stash?

Personal Parts Stash (PPS)! Every 914 owner should have one!

Yeah but you said Store Some of my best HTF parts came from Stores just like that.. Just messing around, Stash is good.

Ironically Brad, your name was mentioned during the process and toward the end. As we were working on things I said to Mark only half jokingly "you wouldn't happen to have Brad Anders phone number would you." Watching Mark work is pretty cool. Toward the end, when my frustration was at its highest - Mark had decided it was probably the MPS. I had a newly rebuilt distributor from Rich Bontempi that was all assembled and ready to stab in the car. Mark let me make the decision on which to do (you should read this like this - Mark let me make the mistake of choosing to replace the distributor even though he had eliminated it as the cause). He is very methodical and you guys in the Bay Area should be using him for your tuning for sure.
In the end when we discovered by serendipity on my part and logic on Mark's part that it was the MPS - Brad's name got mentioned again. Our assignment now is to read Brad's site on MPS adjustment.

Thanks to Brad for the information you have put together and thanks to Mark for expertise you put to work on these cars every day.

I'm just glad it's running better. There's nothing I love more than another 914 on the road.

I just got done reading Brad's section on tuning the MPS. I wonder if we could coax Brad into coming to WCR to do a tech session on this...

Just as an attempt to save Mark's hide I should point out that he loves Allison MORE than seeing a running 914 on the road, he just forgot to say that

When the new MPS units were discontinued by Bosch we had to solely depend on Brett Instruments now Fuel Injection Corp, to rebuild the old ones. Without going into the intricacies of the units like some of the guys now have done (and praise to their efforts) we never broke loose the glue from a rebuilt unit except in the case of a dissimilar engine, to adjust the units. Usually when the rebuilders set them to the so called factory specification, they work just fine on an engine without any other problem, and yes the first installation usually means they go from rich because of the old broken diaphragm sensor, to lean with the new one. We have been able to correct this by returning a bunch of other problems to factory specs, like curing vacuum leaks, replacing lean running injectors, replacing temp sensors adjusting fuel pressure etc etc., working around the mps and using it as a constant. We of course have the luxury of substituting components and found that very few times if the rebuilt pressure sensor did not make the engine run properly, that any other substituted would make any difference either. It was always another problem.
We have found in the past that way too many so called "mechanics"; would take apart the MPS., dig out the glue, and adjust it until they made the engine run better. This operation would do nothing but compromise the customers car by covering up another problem and was NOT a solution. Again I stress that when dealing with a factory d fuel injection problem, it is imperitive to work with constants to cure that problem and not wing it adjusting one to compensate for the other.
Please note that I agree that there is not substitute for brand new factory components and that many rebuilds are compromised by inferior replacement parts, but that is really not the route of the problem. In our shop we NEVER depend on just one example of a replacement component upon which to base our diagnosis and repair. ( I realize that everyone does not have that luxury!)

Good comments from George. RE: "factory specification" - that's part of the problem here. Before Bret sold their business, I spent some time on the phone with Don Burdenhart (owner) talking about their procedures. Don built a D-Jet simulator, very much like the tester that I have, that let him measure the injection pulse width as a function of vacuum. He used a set of "new standards", NOS MPS's, to build calibration charts for each model, and that's part of the problem...

While nearly all of the MPS's (certainly those for the 914) have the same part-load adjustments, the full-load adjustment varies as per the displacement and motor type. That's the main difference between the 037, 043/044, and 049 MPS's. What I've found is that there is significant variation in the full-load adjustment across a set of NOS MPS's, as much as 0.4 ms. Additionally, I've found that even though the part-load setup on an MPS doesn't vary much, the exact setting required for optimal running DOES vary. In other words, you can install an MPS and the car will run fine, but that doesn't mean you're not running at 4.0% CO or greater under part-load conditions, and wasting fuel or failing emissions. The MPS is very sensitive to changes in part-load, as I mentioned before. Even a 0.1 ms variation is very significant, and variations of this magnitude are not uncommon. From data I've gotten from Jake Raby and from my own measurements, you want the part-load CO to be between 3.0 to 3.5%. I've gotten some good results even at 2.5%, but I haven't verified this isn't too lean under hot running conditions.

Like George says, if you don't have the gear and understanding to do this kind of testing and adjustment, then it's more likely than not than not that you'll either mask an existing problem or make things worse. I've outlined some of the procedures on my pages, but over time, I hope we can come up with simpified procedures and perhaps services that can help owners get the most out of their D-Jet cars.

Good information from both George and Brad...

The problem returned the very next morning (yesterday). I decided that since I now had a good running 914 and the sun was shining I would take a decent drive.
I started her up and she idles fine. I adjusted the idle down a bit as it was up around 1600 and you could hear the air sucking past the bypass screw. Once she was warm I took her down the driveway and turned on to my street and as I eased into the gas I realized the problem had returned. Lean miss and backfire. So back up the driveway we went and I hooked up Mark's LM 1. At idle I have stoichiometric mixture. At any tip in at all, I have 22:1 until about 3/4 peddle and then I am back to just a bit richer than stoichiometric.

Well, what to do now. In theory, everything should be right but something is wrong. The MPS is a constant as George said as I am not going to open up Jim's MPS. It is still actiing like TPS so I replace it with another and there is no change.

Well, now we have replaced everything but the brain and the temp sensors I and II.
Temp sensors shouldn't cause this so I swapped out the brain. No luck.

Being an academic I have decided that since this problem has defied all logical solutions it must not be a problem. I am imagining it.

Then, I discovered that with the key on turning the throttle doesn't result in any audible clicks from the TPS. I suspect though that the problem is not the TPS as three different TPS changes have not altered the problem. It has to be something that effects the TPS. I set up the harness grounds remote to the engine on the rear firewall. The question is, do the grounds have to be on the engine?

Have you remembered why you had carbs. yet?

I sure have Al. But remember, this is Jim's Dwight Mitchell Motor and as far as I know, his goal is to keep it DJet as it was designed. It is certainly an admirable goal too... I really like the way you can reach in through an open window and turn the key and have it bark to life and idle and warm up so nicely. If the problems can be worked out great.

I doubt I will ever have a DJet car again as my new Raby Kit Motor will be just under 2.4 liters and use SDS with Jenvey Throttle Bodies and crank fire ignition.

Maybe a stupid suggestion but have you tried a different fuel pump?? I had a similar deli ma with my 2.0, started throwing parts at it checked and re-checked everything a dozen times, fuel pressure always showed 29 psi. Car would run rich then lean out and backfire.I finally just gave up and tried to drive it, then one day it just died, would not restart. The pump kicked it. New pump solved all my problems and has run like a champ since. I though fuel pumps either worked or did not....mine died a slow death.

Your experience points out the issues with diagnosing problems on these 40-year-old cars. At this age, just about every system is suspect and has to systematically be checked out. If you'd done a fuel supply test (measures how much fuel the pump can deliver in 1 minute), a test you do BEFORE checking out the FI system, you'd have caught this before throwing expensive FI parts at the car. I've got some checklists on my site on how to go through the car systematically to catch such problems.

Issues like this are why I'm always reluctant to diagnose a specfic problem over the internet, without having seen the car or seeing all the checkout data that was done before going after the FI.

Which is exactly why I think you ought to jump on a Southwest Airlines flight. You could be here in 3 hours...

FP is a constant 34psi... Have not checked delivered fuel in 1 minute though... I have the fuel line guaged with a pressure guage.

Well, we finally hit that doggone mole squarely on the head. And not a moment too soon either as I was about to call Jim Dupree (jcd914) and say 'bring your carbs. I give up'. But today we employed serendipity rather than logic and found the problem.

For those of you who are 'Webster' challenged, serendipity is defined as a happy accident. We had one.

Up until now, I have employed logic to figure out why the motor runs lean off idle. Figure out what the variables are and eliminate them one by one. Using logic, this problem cannot possibly exist as all of the variables had been eliminated, some multiple times as the possible cause.

Today, Jim and I were going to switch out the ECU for an early one as my Bowlsby harness is an early one. We thought perhaps there was a harness to ECU incompatibility. That didn't work but Jim fortuitously decided to try a different MPS. I decided to entertain his whim even though I had eliminated that as the cause multiple times. Here is what he discovered by serendipity; when the MPS is mounted on the tin as it was in the case of this engine instead of held off to the side in close proximity to the normal mounting spot, the engine misses and goes lean off idle. On the other hand, when the MPS is held off to the side close to its normal mount location, the engine runs like a top.

We reason that there are a couple possible reasons for this and they are important to note for others who might have the same problems. First, it is possible that when the MPS is mounted on the tin close to where the coil on later cars is mounted, the EM interference from the coil or distributor might mess with the MPS. Second, we are grounding it to the tin and causing an electrical problem. Lastly, the vacuum line should not be coiled up but rather a straightish shot to the MPS.

I teach logic and employ it as much as I can but I will take serendipity if it results in a great running car. It did.

Years ago, I mounted the MPS to the engine tin (using the coil mount holes) when I set the engine up on jack stands in my drive way to test run it. Since Rob's car had a carbed engine before we stuffed this one in we just left the MPS where it was. At this point I don't remember the order of changes we made by chasing what started as a flat spot at upper/mid throttle.
It would be my guess that it is EMI from the ignition was causing the off idle lean miss problem but I don't know for sure. It may have started when we added the MSD unit (higher & more sparks) or maybe when the plug wires were replaced (routed closer to the MPS) or when the EFI Harness was swapped.
At one point we swapped in a different MPS and it ran great till the next day. Maybe we just laid the MPS in the engine compartment while we went on a road test and then mounted in the same place as the old after road test, thinking we had solved the problem.
Today I tried a different MPS I just happened to lay it off to the side while we were trying to get a mixture reading and the engine ran well, fuel mixture was a tad rich now. Then I laid it on the engine tin and the running went to crap again. Lay it to the side, runs good, on the tin, runs bad.

Sometimes it is just simple things the lead to the problem.

Jim

This is exactly what happened that day with Mark. You swapped in the different MPS and we test drove it with it laying off to the side. Then, thinking we had solved the problem, everyone went home and I bolted it back in place on the tin. The next day it sucked again just like before. It never occurred to me to backtrack that one step.
After Jim left today I took a 20 mile drive up through Folsom and back down the Hwy. 50 to Sunrise. It ran great... I am loving it.

Very interesting result! From what I can tell, orientation has no effect on MPS operation. However, proximity to a "leaky" (EMI) ignition wire could definitely affect its operation. Any induced voltage glitching could affect the ECU circuit it feeds to, which is essentially a monostable multivibrator circuit, which is triggered at a specific voltage level. Issues with ignition wires running near the CHT are also a known problem. You may want to experiment with changing to some new ignition wires to see if there is any effect. Regardless, very glad to hear that you've solved the problem!

Yippee!!

'Tronics..

Aren't they wonderful. Electomagnetic and radio frequency interference can really drive you nuts.

Wires make wonderful antennas and can "connect" with other electircal signals by induction and crosstalk without shorting.

If you suspect it, get out the roll of tinfoil that you use for your tinfiol hat. Wrap the wires with the foil to block the crosstalk (mini faraday cage). Best is to ground the foil. That might provide a clue as to where to go next.

Worked on a system for the military. We would open the metal cabinet door that contained the electronics, tweak and tune it to work flawlessly, and then watch as it went bonkers when the door was closed.

Open the door, worky,
Close the door, no worky.

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