Fuel Injection Issue!

[raygreenwood]

That makes sense Brad. Never the less, because the CHT does drop to 70 ohms..or can, that is generally why I ballast to the reading right around 200 ohms during hot weather (about 125 ohms).
With high ambient temps in Texas and Oklahoma, the air temp sensor can quickly stay maxed out, and is not enough to correct the mixture. The problem is the metal thread installed in the steel manifold. Like everything else, it banks heat, and stays at one temperature regardless of ambient air passing over it. I have been experimenting with an insulated bushing for it to fit into, but I havn't had a chance to actually run it yet.
Depending upon how tightly your system is adjusted, "mucking" with the CHT has agreat deal of affect on the fuel mixture and running. Its not a tool to find extra HP, but if its control levels are not in synch with where your system is adjusted...it will lose you a bunch of the HP your engine should have. In extreme cases, you can damage the engine itself.

The factory stock levels of adjustment of the MPS, and the factory control levels of fuel pressure are not always, enough by themselves in my experience, to always offset the stock "vagueness" of the CHT, especially in the range of temperature extremes where I live and drive. I agree, most fine running adjustments should be done with the MPS. When that adjusting process begins to be pushed out of upper end enrichment range, it should be done with fuel pressure. All of this is assuming that, the CHT and the ambient air temp sensor have not been already pushed to the end of their useful adjustment limits (which you noted was 100 ohms for CHT).
I have found many people adjusting their fuel mixture and mps response, with the CHT maxed, when in reality, a reading of a 100 ohms or so higher would more accurately reflect the current ambient air temps, head temps, oil and engine temp. This would falsley reflect that the engine is running at maximum warmed up temp (say...350F head temps, 185F oil,)...when in fact the head temps may only be 275-85F and the oil at 150F or so...with an ambient of say 85F.
That means that if you adjust things to run perfectly at this range, and the CHT is maxed, simply because the heads ARE past 210....when you actually do reach head temps of 350, oil of 185...and ambient goes up a hair, you will be too lean.

It have found makes much finer adjustment to give a fixed extra amount of resistance to the CHT, before you adjust the MPS. It slows the process of the CHT maxing out, down a little. The only reason I have been piddleing with a variable version of this, is because I have found that at the two main temperature extremes in this region, it works better to have a winter setting and a summer setting for the ballast....and because as you noted, having too much ballast (which would pump the resistance up over 400 ohms) may give difficult starting in cooler weather due to being over-rich. My personal engines don't have that problem with the extra ballast. Partly because of much better than stock injector balance, partly because of better than stock fuel pressure stability and partly because of careful tuning of the MPS. Most stock tuning tended to idle a hair rich. Add to that cool weather and extra ballast,and most stock tuned engines will effectively flood.
On note....having a much better ignition than what the car came with really helps all of this. Ray

[pbanders]

Personally, I haven't needed to do the things that Ray describes to get my D-Jet T4-based 914 to run well. Mine is set up stock, and the idle CO is set to factory spec. The only thing that is non-stock is 50 ohms of CHT ballast resistance I added to eliminate overrun light backfiring when the motor is cold (< 2 minutes after starting) - but I probably could have cherry-picked a CHT and got rid of the ballast resistor.

Like John Larson on Rennlist (a respected Porsche mechanic in SoCal and known expert on D-Jet) said, when these cars were new, they ran great under all conditions. The stock setup on a fresh motor works great. What I usually find when someone comes to me with a driveability problem that they think the FI is to blame for, is that there is some other underlying problem. Tired motor (low compression, low idle vacuum, sticky valves, etc.), malajusted motor (valve clearance), ignition problems (due to badly worn components, ancient wires, cracked rotors and caps, etc.), vacuum leaks (rotten hoses, cracked plennum, etc.), improper parts (I've seen some FI systems with 2 different injector types, incompatable parts from multiple cars, etc.). Once these problems are fixed, the "fixes" previously applied to the FI system come off and the car runs the way it was supposed to.

Bosch did an excellent job of engineering the D-Jetronic system to handle a wide range of operating conditions. When things are right and stock on the motor, it works fine. If your motor is old or modified, then a careful understanding of the engineering of the system will help you identify where changes and/or repairs are needed.

[raygreenwood]

Yep..I'm right on with that. I think the D-jet system was an excellent piece of work.
On tuning issues, I too can get almost any D-jet system to run quite well in almost any condition. I have no problems with the system. Its my favorite to play with.
The reasons why I have changed my tuning over the years, is because compared to how well my engines run now....baseline D-jet tuning was just adequate. Does that mean stock tuning does not run well? Of course not. It got good gas milage, good throttle response and was dependable. But its capable ...with some tuning, of running MUCH better...cooler and cleaner.
Bear in mind, that my last few engines and current engine were not really stock.
Carefully ported heads, higher compression, 42x36 valves, web #73, carefully modified rockers and oil system, modified stock distributor, pertronix kit, MSD system, higher flow Ernst exhaust, modified fuel supply system and lots of little injection tweeks. (this is a 1.7L)

Brad is correct that you do not have to do any of fuel system balancing or extraneous tuning to have an excellent running D-jet system. That does not mean that doing the extra tuning work cannot greatly improve things. Ray

[C]

Here is an update:

I found the CHT and check it (2000-3000k ohms at 68 degrees). It has no resistance. It has a short. I read Brad's info on the CHT and what can happen if you get a short and that accurately describes my car's performance. I have ordered a replacement and will follow Brad's instructions on replacing it. I did not check the MPS yet.

I will replace the CHT and drive it to see how it performs.
I will update once I have changed the sensor.

[Dave_Darling]

Be REALLY REALLY CAREFUL when removing the CHT sensor. it is ludicrously easy to strip the threads in the head. (BTDT!) There are a lot of strategies for removing them, and I can't say which work and which don't. I know some people who will only remove them when the motor is dead-cold (aluminum is stronger then), and some that will only remove them when the motor is fully hot (the aluminum expands more than the steel sensor).

I would hope that a good penetrating oil (Aero Kroyl or PB Blaster--WD-40 is not a penetrating oil!!) would help here, but I'm not sure. Spray once a day for about a week, and then see if the sensor comes out without its threads being packed full of aluminum....

--DD

[pbanders]

How easy/hard it is to remove your CHT depends on how it was originally installed. They don't need the force of God to tighten them, but you'd think so from the way some of them are installed.

The best trick I've used for R&I of the CHT is to buy a deep 13 mm socket and a cheap extension. Use a Dremel cutoff wheel to remove one of the corners on the extension, so that the wire for the CHT can pass through.

The CHT has a copper washer for good heat transfer. To keep it from falling off (as it inevitably wants to), I use a couple of small drops of superglue to hold it on the CHT during installation.

[raygreenwood]

Hee hee...yep, my CHT socket is actually the strangest tool in my box. Mines a little different. I have a deep socket that I just ground a notch in the side of right below the ratchet hole, for the pigtail to stick out.

You can use a fine thin brushing of aluminum based anti-sieze on these. Aluminum only...the permatex is good. I have found that it has no changes to conductivity or electrical reistance, but makes the sensor easier to remove. Beware...use a VERY thin fine coating. You do not wantg to get any on the washer, and you do not want to hydraulically "pump" the blind hole it threads into. You can crack it. So, screw it in slowly to let the air escape. Ray