We've got a 1990 3vze pickup with a camper, excellent condition with low miles from highway and State/National parks. About a year ago now, in the driveway, it didn't start. Well, over the year we've learned a lot thanks mostly to yotatech :-)

We've narrowed the problem down to something related to the IGF (we think). We've jumpered the CSI (cold start igniter) and it'll run. That means that all the distributor coils work, the igniter, coil & distributor do all the spark stuff. (We've metered all that stuff anyway.) The fuel pump works and pressures up the lines. All the wires are continuous from & to the ECM. However, we just don't have any injector action. Noid lights do nothing. We have confirmed continuity of the injector wires to both the ignition switch & the ECM. And we have good +12v on both lines. The ECM injector lines just ain't pulsing to ground. We checked all the grounds and they're all good (not that injectors rely on the head being grounded).

So, while we know the igniter is doing all the spark stuff well, from what we've learned, it's supposed to send some signal down the IGF (I Got Fire) line to the ECM whereby the ECM should pulse the injector lines to ground. So, we suspect the problem is somewhere along that route. We've got three igniters and two ECMs that all produce the same symptoms.

Well, we are officially quite befuddled. What are we missing/overlooking? Is there something else that tells the ECM to fire or not fire the injectors? Please... if anyone has some insight/pointers to help us get our IGF F'ing again... we would be so grateful!

Thanks.... Norm & Tom

 

Have you confirmed continuity between IGF on the ignitor and the corresponding terminal on the ECU? Could be a simple as a bad connection. Check for lack of continuity, and also check for the line being shorted to ground. Either condition will cause your symptoms.

 

X2.........check your connections of the IGF wire and grounds and the plug especially. If you are checking for continuity. Don't probe at the bottom of the ECU plug. If you did, you may need a new plug. Your ECU is fine if it is for the correct year. Check the wire connection at the Igniter plug as well. These sometimes have a loose wires/connection as well at the plug. Probe the back of the ecu using a thumb tack to the wire connector inside the plug to get a volt reading off of, to the actual bottom of the pin of the igniter connector of the igniter for the IGF wire. Use a code reader to find the correct igniter that isn't throwing a code to find a good igniter. GL

 

This might give you some ideas: https://www.yotatech.com/forums/f116...gniter-287857/ Specifically, you can't test for IGF with just an LED (I'm guessing that "igniter testers" could use an op amp, but I've never used one so I'm only guessing.)

You can check for the plugs firing with a simple timing light, with the inductive pickup on each wire. But it sounds like you are past that.

I agree with the others; I'd start by checking the IGF wire from the igniter back to the ECU. Note that even moderate resistance to ground (like 1kohm; you want infinite) is enough to wreck the IGF signal.

 

IGF (I got fire) I lol'd. Have you messed with your distributor? Your CPS (crank sensor) is in the dizzy. Have you done anything with that recently? I think CPS mostly controls igntion spark, but worth a mention.

 

Yes, yes and yes. We just ran out there and did it again for the third time!


Yeah, we've been doing the back probing thing with stick pins and the like. We're getting good and appropriate readings every time. We went ahead and did the diagnostic thingy again (jumper two terminals and read the flashing check-engine light) and again, it's clean as a whistle with no codes.



Wow... that post was quite helpful. It filled in some of our gaps of understanding such as we now know that IGF goes +5v-ish pulsing. (We wondered which direction it went.) Now we can target our signal seeking better.

We haven't messed with the dizzy (that sounds scary though!!). We checked it all out coil and continuity-wise but we're not too concerned about that right now since the engine runs well with the CSI jumpered. (I.e.,. the dizzy pickups, coil and all have to be working in order for it to run.)

It feels funny checking some of this stuff since the truck is in such good shape. There is no corrosion and everything is so clean and tidy relatively speaking. But, we've checked these things again and again.... We have no shorts to ground anywhere, continuity on all wires, connectors are in great condition, no trouble codes, etc. I guess we'll look at these connectors with a microscope. Is the IGF the only thing that gives the ECM permission to fire the injectors? What are the odds of having 3 igniters having the exact same IGF problem?

Thanks for the help guys... we're not giving up yet, hoping there are some more ideas out there... although, it makes a nice spare bedroom for guests!



Norm & Tom

 

Scope103, been thinking more about your other thread for igniter testing and thought we might should try some LED testing. You did say " Nope, your LED won’t work!" but that was in the context of a running engine with the LED killing it, right? Would the LED work on the IGF line while just cranking? We haven't snipped any wires open yet but if we clipped the IGF line and put the LED on the line coming from the igniter (and ground) shouldn't we see a flash when cranking?



Thx....

 

I don't think so (but I don't know). The sense I got was that the IGF from the igniter was a high-impedance signal (like the NE signal from the distributor) that the ECU amplified. By attaching 1k ohms to ground, it overwhelmed the signal and the ECU didn't have anything (enough) to see. (1k resistor means your led is getting about 10ma; you won't be able to see it with much less, so using a larger resistor probably won't help). I don't see any reason why cranking would be different than running (the reason I cared was that the engine turns over so much more slowly cranking vs. idling, so it is possible to see the flashing).

You can always try it. In theory, you should be able to remove the pin from the connector, rather than "snip" it. I tried (for other reasons), and I was unsuccessful. Your choice.

 

Hey y'all... no real update at this point however, I did bump into some local guy who has a fair bit of knowledge and access to good info. He mentioned that the only time the CPU will operate everything except the injectors will be in a DFCO (deceleration fuel cut-off) condition. He looked up this truck and determined that this '90 3VZE did have that feature. Best we could figure is that two conditions need to be met for ECU to briefly cut off the injectors. 1) some speed say over 40mph, 2) throttle from a position dropped to zero/idle. Hmmm... anybody know anything about DFCO on these vehicles? TPS sensor and some speed sensor? What else might play into this scenario?

Still grasping... :-\

 

All I can say is "he is correct." I've got one of these: https://www.yotatech.com/forums/f116/mpguino-283777/

and yes, rolling down a hill at speed, the injectors will stop firing.

But that just doesn't sound like your problem.