Measuring running voltage to the injectors might be a good idea. I purchased a 3.2L for my 914 project about 2 years ago. The wire harness and electrical connector boots had seen better days so I decided to replace all of the electrical connectors and put a new outer jacket on the harness. When I tore off the old jacket, the two main power feeds for the fuel injectors near the DME relay where the main feed from the relay branches in 5-6 wires had melted insulation and one of the leads only had a few strands of wire still attached. It looked like there had been a short in the system at some point and this area had been a point of high resistance that had overheated. If I had not opened up the jacket, I would have never found this damage. With a few strands still attached, the system might still have functioned but with greatly reduced current capacity.

I completed the install of the motor this past weekend and will try to fire the motor tonight. Wish me luck.

That's a good thought. I just bit the bullet and bought a new one, they are only $10! Sometimes the o-ring is damaged, so this includes the o-ring too...

-Wayne

Okie, I got the low down on these. It seems that these non-resistive plugs appear to be quite difficult to find these days, as they were discontinued about five years ago or so. This is the story:

W7DC - discontinued about five years ago, replaced by W7DC+
W7DC+ - also discontinued several years ago, replaced by W7DC0
w7DC0 - also discontinued several years ago, superseded to the 7900 series plug, which is the WR7DC+ plug, which is the one with the resistor and the only one available from BOSCH right now.

One can try to run a slightly hotter plug, like a W8DC plug (or W8DC+ or W8DC0, which are also all discontinued).

NGK - also doesn't have a replacement plug without a resistor, as far as my sources have indicated.

There is supposedly a Champion plug (don't know the part number) which is supposedly a replacement. These are cheap and available, but I don't have the part number for them. This page *may* help, but there are many resistor plugs included there, so some research would have to be done: https://www.sparkplug-crossreference.com/convert/BOSCH_PN/W7DC

I've been told that if you can hear a buzzing in your radio when you rev the engine, then that most likely means that you have non-resistive plugs and wires in your car. My parts sources implied that the resistors may have been installed to minimize electrical interference in the car's chassis. Not sure about that one, but I thought I would mention it...

-Wayne

Good thought. However, I already swapped out the harness with another one that I borrowed from Steve W. - which made no difference. I thought for sure that was it too!

-Wayne

I have checked quite a few times, and there are indeed some minor electrical bugs on the car (voltmeter in the dash reading low, etc.), but for the most part, the electrical system is fairly solid. Good grounds, nothing too wonky that I haven't fixed already by cleaning contacts with a wire brush (like the cigarette lighter reading 10.5 volts - that got fixed with a wire brush).

-Wayne

Yup, got that checked out multiple times. The spring was a bit weak (the one that pushes the throttle back against the switch), so I replaced both springs.

-Wayne

Yup, a while ago Ingo suggested that I wire the injectors directly up to the battery +12V on that end, instead of running it through the ECU. I did that with some beefy 14 or 16 gauge wire - didn't make a difference. :(

-Wayne

Might as well throw some new injectors at it. Maybe a flaky injector coil? I know they've been cleaned and flow checked but If you've verified voltage and even gone as far as changing the injector harness along with new plugs, different afm, new fuel tank, new CHT, new fuel rail lines, verified fuel pressure solenoid is working properly, changed out DME and a laundry list of other things, it seems the only things left are a bad wire in the main harness that connects to the injector harness or an injector coil failing.

Beyond that, all I can think of is "Damn!"

Hi Wayne,

I think you still owe the yourself and the group a detailed measurement directly across one injector with an o-scope. Do it with the following conditions:

- stone cold (will tell us pulse length increase due to CHT input with engine cold)
- at operating temp with problem present (want to see pulse length)
- at operating temp with problem gone because other bank disconnected.

You might even go as far as adjusting the idle mixture screw in the AFM and confirm this will change the pulse length at idle as designed. Remember, that screw admits unmetered air into the engine that isn't deflecting the barn door. To richen things up close the screw all the way.

Beefing up the +12V supply to the injectors is only one side of the equation. The GND path comes from the DME through the DME wiring harness and that could be problematic, too.

In the end it is important what each injector "sees" and whether there is a discernible difference in what a particular injector sees with the one bank disconnected.

Cheers,
Ingo

Wayne..i have mentioned earlier the spark plugs W4CC which are used for non CAT 3,2 engines ,you cannot get those?If not i can send you some...

Ivan

Kudos to you for banging your head on this for so long, at this point even if you were going to scrap the factory EFI (which I will be doing on my hotrod build) I would have a need to get to the bottom of this as my OCD would not let it go !.

I have been trying to find the specs for the bosch injectors on this engine, but no luck. I believe these are low impedance injectors which would mean they need a lot of current to be driven properly.
In post 121 you measured the resistance, checked the voltage drop and even replaced with a test harness and another DME. But the issue persists when both injector banks are connected.
Have you thought about checking the actual current at each injector ? since you have a scope, you could use something like this:
https://www.picoauto.com/library/automotive-guided-tests/injector-current/#:~:text=Plug%20the%20low%2Damp%20current,wire%20a s%20in%20Figure%201.

I don’t know if you have attempted this yet as I have not read every page..apologies in advance.

Ingo,
If it was a ground path issue related to the harness, wouldn't that tend to be more a constant problem than the intermittent problem he's seeing where sometimes it runs fine and others not?

Nick, yes and no. If (and that is a big IF) the injectors really see something different are not precisely open for the short idle pulses when all six are firing it could be anywhere in the DME harness, the DME connector, etc.

Wayne has test-swapped DME (so let's rule that out), the engine harness (ruled out) and routed alternative +12V to the injectors. However, if there is say a corroded crimp in the DME harness that would do it. Also a faulty injector can stress the current-regulation inside the DME to where the idle pulses are becoming somewhat garbled. A set of careful scope traces across an injector and ideally across the current-sense resistor inside the DME would give clarity if there are losses somewhere.

Remember the entire system receives GND from these ring-lugs at the end of the DME harness that mount to the engine bay GND point. Internally there is a pretty wild crimping in the DME harness with different GND wires when you open it up.

I really don't think we have definite conclusions as to whether the issue is now better than at the onset oft the endeavour. I have see too many emails with "no change" and maybe some of the observed "improvements" are wishful thinking. It's hard to image you had two root causes at the beginning that result in the exact same issue and now one is addressed the other is intermittent.... Orcam's razor says otherwise ;)

Agree. BUT - I was looking at the problem solely at idle, and thinking, "well I'd better fix that before driving the heck out of it - if it runs like crap at idle, it will run worse under load". That assumption is not true however, the car seems to run very well when not at idle. So, the issue isn't better, but the scope of the problem seems to be limited to idle when the engine is not fully warmed up yet?

The number one culprit that would explain all of this would indeed be an air leak on one side of the motor. That would explain running fine when cold, running rough when a bit warmer, and also the intermittent fixing of the problem at hot (it would be sealed, unsealed, and then perhaps sealed again). It would also explain the mixture differences left-to-right. That would also explain the O2 sensor making the car go lean (as it was sensing a richer condition on bank 1-3). Alas, I have not found any air leaks *anywhere*.

???

-Wayne

Last weekend I finished to built a 3.2L 911 Carrera engine into my 914 (same as Wayne). In my case the engine ran fine when cold but started to run erratic after a few minutes (stationary as well as at high rpm). It turned out that the fuel line to the filter was hitting the valve cover and therefore became warm, probably resulting into vapor in the fuel system. After fixing the fuel line with a tire wrap to the chassis of the engine bay the erratic running disappeared.

My apologizes for hijacking this thread with another problem related to this engine and the 914. At full throttle and high rpm (> 4000) my clutch is slipping. It concerns a 901 gearbox with a KEP stage 2 (215mm push) pressure plate. When I had a 3.0L engine in this car I used a KEP stage 2 225mm push pressure plate, but this was way too heavy for normal street usage. Any ideas or suggestions?

air whine / whistle noise
 

The air whine / whistle of your engine at the video from your message at 05-13-2020, 08:53 AM is identic as mine (using K&N cone air filter). So I think the whistle noise is normal.

Wayne,

In summary your problem statement is “poor idle when the engine is warmed up”, right?

There are several things happening when the engine transitions from cold to warm mostly related to mixture:

- fuel absorption and condensation on intake and head parts gradually reduces
- atomization of fuel mist improves
- ability to generate a stable mixture improves
- internal friction reduces
- temperature of everything rises. Interesting observation from ggraf there.

The CHT is accounting for that by gradually reducing the enrichment. It’s enrichment authority is very significant when the engine is stone cold.

Starting here I remember it was suggested several times to play with the mixture at idle and see if that makes it better (fuel pulse duration from DME). Maybe try any of below when the problem occurs:

- gently push the flapper of the AFM a bit more open
- close the AFM idle bypass screw all the way
- hook up a 5k logarithmic potentiometer instead of the CHT

All the time measure the pulse length with your O-Scope. The potentiometer is a bit more work to realize but it makes it very convenient to take data points while observing the O-scope. The fuel pulse is on the order of around 1ms.

A decade switch resistance box is really easy to connect and use for test. Its also very cheap.

Along the same lines of isolating the CHT for any potential contribution to the problem: Would it make sense to mount a CHT sensor on a temporary bracket somewhere on the engine that does not have as huge of a warm-up temp delta as the heads? Would be interesting to see if a more consistent CHT changes the behavior. This way it's not just the sensor that is being tested, but rather what the ECU does with the reading as well.

I believe Wayne replaced the CHT sensor already and excluded this as possible root cause. If he has the old one it's the perfect parts source with it's harness. Simply buy that 5k potentiometer and wire it up to the pigtail of the old CHT. This makes it a simple tool to modify the mixture in a very wide range.