I seem to have an issue with my injector trigger points on 72 1.7L. All 4 cylinders seem to be running a little rich which I think is another issue, but 2 of the diagonal ones (2+3) are super rich, they totally blacken the spark plugs within a few minutes. I pulled the distributer and cleaned up the trigger points with some contact cleaner and they looked fine to me. It seems from what I've read that they are double firing? What would cause this? Wondering what I should be looking for when I pull the distributor again, and how I can test them once removed? They have 60k miles on them, would a new set be worth the $$? thanks!

warm it up a bit and try grounding the lead to the head temp sensor on the neg terminal of the battery. I assume first that you have checked the fuel pressure and evacuated the pressure sensor and it holds vacuum????
(of course you have checked the dwell and the timing and they are spot on and the valves are in adjustment and the compression is even!!!!!!)

As for the testing, I agree with George. Effective testing requires fundamental parameters to be at base. That means reasonably even compression and correctly adjusted valves.

As to the "double firing" issue, the first I've ever heard that brought up as a potential problem was yesterday, or the day before, in another thread. I've never seen it on any D-jet car. That doesn't mean it can't/doesn't happen, just that I've not seen it in 37 years of working on D-jet cars. I've asked a couple of other ol'pharts in the biz, and they've never seen it, either. FWIW, the most common failure mode of trigger points is that they fail to make the necessary contact and either 2 or all of the injectors don't fire.

The Cap'n

Yes I checked the other stuff, fuel pressure, valves, vacuum leaks, new injectors, even compression, timing, etc.. My MPS does hold vacuum, but not up to specs on pbanders site. I pumped 20in/HG and:
down to 15 in/hg in 16 seconds,
down to 10 in/hg by 50 seconds
down to 5 in/hg by 2 minutes.

My MPS part # ends in 041, I also tried out a 049 one (supposedly should work with 1.7????) that a forum member sent me to try which held vacuum considerably better than mine but pretty much rendered the car useless. Lots of popping, erratic idle, wouldn't rev over 3k, no power.. I put my old one back in, since the car will drive and run smooth with it.

Anyways, wouldn't most of that stuff affect all the cylinders evenly? I was hoping to get all 4 running the same first, then start looking for a replacement MPS. What would grounding the cyl. head temp sensor do? Seems like it would affect all 4 cyl. the same? Is there something I'm missing that would affect only 2 cylinders?

I remember reading about the trigger points double firing somewhere, but I can't for the life of me find it again... Only what you said about them not firing at all..

ok, I just found it, it was on pbanders site:

Failure Modes:

* "Bouncing" or dirty contacts: A "bouncing" contact causes multiple injection pulses to be generated, resulting in a very rich mixture. Can only be checked by using an oscilloscope.

wondering if this would be a problem with the actual trigger points or the cam that opens/closes them..

If the problem is truly in the trigger points, and your wiring is correct, you should have problems with opposite cylinders, not diagonal ones. Be sure the wires are in the correct places (if the boots are still there, both gray ones to one opposite pair, both black ones to the other pair). As the injectors fire twice per full cycle, the position shouldn't matter all that much, but they should be right anyway, just 'cause. Just for grins, swap the wires fore and aft and see if the problem moves with them. Another thing you might try is to pull one injector plug at a time and check the RPM drop. Wait 15-20 seconds after plugging one back in before you try another, to allow the engine to stabilize.

One thing we haven't addressed is the possibility of a nasty spray pattern of the 2 injectors that seem to have a problem. You'll have to pull them, jumper the fuel pump relay, and trigger the individual injectors, one at a time, and looking for a nice conical fog spray pattern. A drizzle, a stream, or other pattern that's not a fog, will cause that cylinder to run poorly, probably even rich.

Let's keep this discussion going, because it's what we're all here for!

The Cap'n

The wiring to the injectors looks right like you said it should. I just tried your idea of swapping out the injector wires fore & aft to see what happens and the problem moved with em.. so would that just leave the trigger points or the ECU as the culprit?

I guess that would depend upon weather the problem moved when you switched the wires.

Have you looked at your cold start injector? I had one that dribbled all the time...I removed the fuel line from it and the electrical line and routed the fuel line around it and the car ran much better...not as rich.

Rich

worn trigger points have cuased rich running on several 914s I have delt with. I have had this failure much more often than then having them be poor contact (ie dirty or corroded contacts) just a fact of life as these cars age. this should be considered a consumable item, yes a long term consumable, but eventulally they all will wearout and bounceif you dont change them

what happens is the wear block that rides on the cam gets too small so that the points just barely open, eventullay the wear will be so great on the wear block that the dont open at all (ie no firing) but before they get that bad, they open with such a small gap (say for arguement sake they only open a max of .0005 inch when highly worn) now they can bounce and trigger double or more firings. if they barely open due to wear on the wear block they will bounce!!!!!!!

the wear block I have found should be considered at their limit when teh small taper on the nose of the wear block is worn away to be flat new ones should have a taper on the tip, wornout ones should be flat.

now if your in a bind money or time wise, you can try bending the points arm a bit to make them open a bit more, then you can get a bit more life out of them.

the symptom tends to be worse at high rpm so that if you bench test them by turning the dizzy by hand and test with an ohm meter they seem fine, but run them up at high speed, and they can bounce.
also when you mount the triger points you are able to , due to clearances in the holes the mounting screws go thru, you can slide the points back and forth a bit before you tighten down teh two retaining screws, I recomend you center them as much as possible before tightening them down. thus you will have a more simular gap on the two points, slide them to one side or the other and one set of points wil have a smaller gap, and hence that one will be more suspetable to failure first.
By hand rotating and watching ohm meter for open/closed contact and noting the postion of the cam (via the rotor) you can get them very centered (set them up by noting the cams rotation angle verse when they first open, then repaet on other set of points. and move the whole assembly back and forth until both sets of points open at exactly 180 Degrees from each other. if they are off center, they will not be opening exactly 180 from each other, and hence are not centered.

very simple to set them up right, this will ensure longest service life as both gaps will be maximized and both cams followers (wear blocks) spring pressure will be minimized for reduced overall wear.

little details like this set up proceedure will give you many many extra miles of driving before failure.

anyone can install a set of trigger points and blindly screw them in place. but your not just anyone, so take the time to get them dead centered.

PS if your points are getting dirty oily often then you may find you have poor crank case venting, and/or a worn dizzy shaft that allow oil / combustion gases to get on them form the crank case correct crank gas venting is more than just smog control, it helps make your car more reliable!!!!!
yes!!! swap the injector wires to test and isolate if it is an injector issue or double injection issue. ECU failure can happen, but is fairly rare, The firing siginal to the two power transistors (the electronic switch that fires them) is triggered by the trigger points. other inputs (such as head temp or mainifoold pressure, throotle postion) determine the firing pulse time, but the actual tirgger to fire is only from the trigger points. the pulse time should control all four injectors evenly. the trigger is the only thing that effects pairs of injectors

also you can test injector flow volume by removing all four and placing the end sof them in four graduated test tubes and crank teh motor for several seconds, then note if they all pump out eh same volume or not. a good source of graduated test tubes in sthe drug store, get the plastic baby liquid medicine tubes, they should have teaspoon of ml markings on them, get four of them and your good to go.
also make sure spray pattern is a nice even cone shape and note if any injector contiunes to weep fuel out when you turn ont eh fuel pump with out cranking the motor. a weeping injecotr will need clean or replace

Randy W from the Roadglue site found a set of bouncing trigger points. He chased and chased a problem for a while and finally dug out a scope to check the trigger point signal. At least one pair was double-firing.

BTW, Cap'n... I believe the trigger points do fire diagonal pairs, not opposite ones. And I think D-jet only injects once per cycle; the L-jet injects twice per.

--DD

Yup, 1 and 4.... 3 and 2 per haynes that is diagonal that is also consistant with the firing order.

i

Well I pulled them out again, and I could actually see them double firing on the ohmmeter on one side while rotating the shaft quickly.. In 1 rotation, the meter needle would bounce twice as much as the other side. I think it's like you said they were barely opening. I guess I'll buy some new ones! Yay.. at least I know I need them now... In the meantime I bent the spring/clip thing like you suggested so they will open up more. Looks good on the meter now. I'll give her a proper flogging tomorrow and see how it goes. thanks for all the help fellas!

welcome, most excellent!!!!!!

Interesting discussion, a couple of comments. First, the the TC's are a simple, cheap spring switch. All such switches tend to have bounce, that's why the ECU has a debounce circuit. See:

http://members.rennlist.com/pbanders/ecu.htm#TL

That said, the debounce limit of the ECU TL circuit is limited, and if you've got a switch that has a really long bounce (probably has a defective spring), you can get two pulses. It's a simple enough matter to swap in a new switch (uh, you DO have a nearly full set of new or "tested good" FI parts on the shelf, right? ) and see if it works.

BTW, normally to see switch bounce, you need an oscilloscope. If you're seeing it on an ohmmeter, then it's really messed up.

Last comment is on the MPS leak-down. I haven't updated my page with this data, but I've noticed two leak modes. When they leak down fast, it's a cracked diaphragm, it's got to be replaced. When it takes tens of seconds for it to leak 5 mmHg or so, the leak is probably coming from the adjustment screws. If you've got an MPS that's already had the epoxy removed around the full load stop, you can significantly decrease the leak by using a small screwdriver and layering some thick grease around the inner adjustment screw. I've done this myself and it seems to hold up for a long time. Regardless, the impact of a small leak on the operation of the MPS is minimal, because the motor has more than enough pumping capacity. Only when the diaphragm becomes cracked does the MPS have to be replaced.

You're right, Dave, about the pairing. I was wrong about that. It's 1 & 4, 2 & 3. However, you're wrong about the number of pulses. Each injector fires twice per cycle.


The Cap'n

Cap'n, please check page 0.1-1/9 in your factory manual. You'll see the diagram of intake opening, spark, and injection in the middle of the page. Notice that there is one injection event ("B") per cycle. For cylinders #1 and #3, the injection event happens at the beginning of valve opening ("A"), and for cylinders #2 and #4 it happens about a quarter-cycle before valve opening.

I think you're mixing D-jet up with L-jet, which opens all four injectors twice per cycle.

--DD

http://www.914world.com/bbs2/index.php?showtopic=105542
I think this is the one. Look for the answer from ROOT WERKS.
Tom

Well, Dave, you're wrong. Each injector fires as part of a pair. That's why there are only 2 sets of points in the trigger points, and only 2 lobes on their cam. Half the necessary fuel is injected while the intake valve is closed, and half when it's open. That makes TWO pulses per spark event. A complete cycle is 720 degrees of crankshaft rotation on a 4 cycle engine. See pages 122 and 123 of the Bosch D-jet manual.

The Cap'n