3.2 question: How to test for impulse to fuel injectors?

[Fairview]

I need your advice. I believe I’m at the point of determining whether the fuel injectors are all stuck, or the DME box is bad.

This is for the 1984 Carrera that has been in storage for twenty years. It lit up and sounded great with a whiff of starting fluid this morning, but won’t run otherwise.

I have replaced the fuel tank and fuel pump, cleaned the fuel lines, installed new DME/fuel pump relay, and new fuel filter. With terminals 30 and 87b jumped per the factory manual, the pump runs and fuel circulates and comes back the return line to the tank. Also, with the relay plugged in, the fuel circulates with the starter cranking.

However, it won’t run at all, zero, except with starting fluid, and then sounds perfect for a couple seconds, so I must be getting no fuel into the cylinders.

I have confirmed that with the ignition on, there is 12 volts to the injectors. I don’t know how to check the other terminal on the injectors, the one which I think is the impulse from the DME. Also, can I check at the DME box? I’ve read the Bentley manual, but it is a bit vague and I can’t tell if my multimeter is the right tool. Can you talk me through it?

I guess it is conceivable that the DME is OK, but that every single injector is stuck from being unused for 20 years. So that is why I want to do the impulse check.

Thanks very much, Jeff

[Solamar]

I know the injectors fire with a ground signal from the DME.

My guess is the injectors are stuck.

I've head you can soak them in acetone and use a 9v battery to actuate them in an effort to clean.

I opted to send mine to mr injector and they came back good as new with flow specs for not much $$

[john walker's workshop]

Noid light.

[yelcab1]

Or a scope, like an oscilloscope.

[Fairview]

Thanks!

[FrankM_]

If you want to test the injector signal, use a scope.
As you identified, one side is 12V, the other side gets connected to ground by the Motronic ECU (pin 14 and 15).
It looks like they apply a peak-and-hold signal :


In any case, you can relatively easily remove the fuel rail with the injectors on it and send the injectors off for testing / cleaning / retesting and the rubber parts and fuel screens will be replaced.

What I am wondering, are you sure it is a fuel problem ? It seems that running 'a few seconds' simply on a small burst of starting fluid into the air intake is long ? Maybe your idle mixture is not correctly setup (not enough air/oxygen ?).
I built myself a breakout box to save me some time in debugging these kind of problems, but it can be done by measuring at the terminal of each sensor too...



Anyways, the official DME test plan calls for the following debugging steps :
1. Test ground connections: on intake runner nbr 1, near the fuel filter, strap between body and transmission

Test all the plug connections (clean, well seated) : 14 pin connector in engine bay, the three connectors on the left side, the 6-pin connector supplying the injector signal, plug on the ICV, idle throttle position switch, WOT switch, AFM plug, connector on each injector, DME connector (35 pins), seating of DME relay

2. Check fuel pump at starting. It should run, if not check fuse
Check 12V power to the DME

3. Check speed sensor output with scope : sinus with amplitude > 2.5V

4. Check reference sensor input with scope : single oscillation >2 V

5. Check iginition signal (to coil) with scope, check wiring from coil to distributor and from distributor to spark plugs. Check distributor cap & rotor.

6. Check AFM power input (5V), check output from sensor plate, check output from air temp sensor

7. Check fuel pressure at the rail test point : 2 bar, without vacuum on regulator 2.3 to 2.7 bar. This 'no vacuum' value should also be seen when you operate the fuelpump without the engine running.

8. Check ICV operation

9. Check throttle idle switch operation / check full throttle switch

10. Check engine temp sensor II (top one of the left three connectors) for resistance
15-30 deg C (normal temps) - 1.4 - 3.6 KOhms

11. Test injectors
If engine runs : pull injector plug one at a time : engine should run rough / rpm drop
If not running : test injector coil ohms : 2-3 Ohms
Check if signal from DME arrives by scope connected to cable plug

12. CO adjust , idle speed ajust

13. Check for air leaks in system & check alititude correction box (bridge)


I am using my old analogue scope in the example above, but I'm sure it can be done with one of those $35 'DSO shell' scopes on amazon since the signals are not really very high frequency.

[FrankM_]

Originally Posted by Solamar

I know the injectors fire with a ground signal from the DME.

My guess is the injectors are stuck.

I've head you can soak them in acetone and use a 9v battery to actuate them in an effort to clean.

I opted to send mine to mr injector and they came back good as new with flow specs for not much $$

Do NOT use a 9V battery to 'activate' them ! The injector can easily be damaged by providing continuous power. It is supposed to be driven by a brief opening peak and then a succession of much lower voltage peaks to keep it open (depending on how much fuel the ECU wants).
Better go to a shop that has a professional testing/cleaning rig or build a small electronic drive that doesn't blow up the injector when you use it too long.

[Fairview]

Thanks for the input. And Frank, thanks for the checklist and other information. This will keep me busy for awhile!

[gdp_samson]

Hey Frank, where in Belgium would you send your injectors to ? I went to a Bosch shop but they sounded really surprised when I asked them if I could bring in 6 bags with a fuel injector for inspection/rebuild...(I'm also on the Belgian porsche forum; gdpsuz1982)

[r lane]

Hook a battery, can even be one out of a 9 volt flash lite, to the injectors and see if they click. some times that is a way of freeing them up.

[FrankM_]

Originally Posted by gdp_samson

Hey Frank, where in Belgium would you send your injectors to ? I went to a Bosch shop but they sounded really surprised when I asked them if I could bring in 6 bags with a fuel injector for inspection/rebuild...(I'm also on the Belgian porsche forum; gdpsuz1982)

I bring them to hvanrompaey.com, since about a year or so they also have a petrol injector testbank. You just need to specify that it is a low impedance injector to be tested at 2.5 bar.
It is not cheap though, incl tax I paid about 32 eur per injector (including every renewable part renewed off course).

They are also a good stock for hard to find parts and rebuild a/c compressors ec...

[GH85Carrera]

Originally Posted by john walker's workshop

Noid light.

This. Much easier and less expensive than an oscilloscope. The scope it cool, but most home mechanics don't have one handy.

[Mark Henry]

Noid light is nothing more than a test light, all FI wires worked.
Turned out to be the "rebuilt" fuel injectors, replaced the injectors with new FiveO units and she fired right up on all /6.

New, but way smaller issue, is the throttle plate isn't returning to zero. It's almost there with the spring I have but just won't go the last 1% and I can hear a solenoid/limiter click when I push on the return.
I'll have to look for a thread on solutions for the throttle return.

[911obgyn]

Check bushings on throttle links under the car.
There are oilite bronze bushings on this site ,way better than the plastic original.