Injectors not firing at low RPM

Series8217 · Post by **Series8217** » Fri Mar 10, 2006 1:19 am

I'm having a very strange problem with my 3.4 DOHC Fiero.
90% of the time when I try to start my car it will not fire. The fuel pump turns on, there is pressure at the fuel rail, spark, the ECM is getting ignition pulses (checked with scanner that it reads the right RPM), and is attempting to fire the injectors (I can see a pulse width using my scanner). However, none of the injectors open. Every once in awhile I can get it to start. It doesn't seem to matter whether its hot or cold. If I push start the car the injectors will fire. It seems they will not fire below a certain RPM (or rather, pulse width, I assume). It is also leaning out under all conditions; this is a problem I've had for a while now but it started getting worse recently. I checked the fuel pressure last week and its fine. BLMs are creeping up past 150 with an equally high Integrator. I think something is causing injector pulses to drop out, but it is somehow more of a problem at low pulse width and RPM. I have wiggled all the wires and connectors related to the ignition system and get no change. I also bought and installed a new ECM yesterday because I figured one of the injector drivers must be bad. No change. The injectors were recently cleaned, screens replaced, and flowtested. They are all within 1% flow rate.
Once the car is started it will generally run okay, though the fuel mixture is certainly off and it obviously stumbles at low RPM. All sensor values (coolant temp, TPS, MAP, IAT, RPM, O2) read as they should.
I'm going to check the grounds tomorrow (already checked the power wire) to make sure they are in good shape. I am afraid they will turn out to be fine.
Any ideas on what else to check?

-Steven

SappySE107 · Post by **SappySE107** » Fri Mar 10, 2006 1:36 am

Aaron · Post by **Aaron** » Fri Mar 10, 2006 1:40 am

SappySE107 wrote:Check your fuel flow. When my pump was going out, the pressure was where it should be but when I disconnected the line and ran it with the test lead (i hope a fiero has this too), you could tell it wasn't pushing the fuel out very well.

Well if the injectors are firing normally, then if the fuel pump wasn't able to supply enough flow the pressure would drop wouldn't it? You'd need injector failure and fuel pump failure.

How about getting it to start, then checking fuel pressure at differing throttle levels. Like rev it up and see if pressure drops radically.

Series8217 · Post by **Series8217** » Fri Mar 10, 2006 2:22 am

Aaron wrote:
SappySE107 wrote:Check your fuel flow. When my pump was going out, the pressure was where it should be but when I disconnected the line and ran it with the test lead (i hope a fiero has this too), you could tell it wasn't pushing the fuel out very well.
Well if the injectors are firing normally, then if the fuel pump wasn't able to supply enough flow the pressure would drop wouldn't it? You'd need injector failure and fuel pump failure.

How about getting it to start, then checking fuel pressure at differing throttle levels. Like rev it up and see if pressure drops radically.

I checked the fuel pressure last week when I was diagnosing the lean condition. It's rock solid at 40psi at idle and briefly bounces up to 43 or so when I rev it and then drops to 35 or 37 when I let off until it the RPMs come back down to idle.
Again, the problem with the no start is the injectors are not opening! Even if I had low fuel pressure and/or flow they should open. I stuck my hand against some injectors and had my buddy crank the motor, I can feel that they are not pulsing. When the motor is running its obvious when they're clicking, you can hear and feel it. There is also no gas smell when I crank for a long time, and I can hear that the motor is not getting any firing events, it just cranks. No gas coming out.

Aaron · Post by **Aaron** » Fri Mar 10, 2006 2:30 am

Yah that completely eliminates the fuel pump IMHO.

SappySE107 · Post by **SappySE107** » Fri Mar 10, 2006 3:28 am

Aaron · Post by **Aaron** » Fri Mar 10, 2006 3:35 am

I believe he just bought and installed a brand new ECM a day or two ago.

w41442 · Post by **w41442** » Fri Mar 10, 2006 8:54 am

injector harness ground could be the problem. When you test the oulses are you using a scan tool, or a multimeter at the injector. If the latter, are you grounding to the ground wire at the harness? If you say the pulses are there, then the injectors should open, but if the necessary current cant flow, then they wont obviously.

bryson · Post by **bryson** » Fri Mar 10, 2006 2:27 pm

Yeah, make sure that the injectors are getting a solid 12v signal. The ECU pulls the ground to fire the injectors, so make sure the ECU is grounded well. Also, what pulsewidth are you reading? Mine won't fire below about 1.13ms, and they are brand new. It seems like slightly older injectors would have even more trouble trying to open for a short period of time, especially if they are a bit durrty.

Series8217 · Post by **Series8217** » Sat Mar 11, 2006 2:25 am

Cranking pulse width was over 6ms according to the ECM. We also hooked up a test light and it flashed, but no injectors fired. Definitely injector power or ground related, since they were getting pulses but not firing! We went over and cleaned/replaced the vast majority of the grounds... and then when I went to test the car my battery was dead. Oops. Too much cranking over the past few days and too little driving.. argh.

After jumping the car it starts but still runs lean until the ECM bumps the BLM up to 154 (I'm running the stock Lumina Z34 chip now, to get PROM issues out of the picture). I don't know if it started just by chance or because we fixed something by going over my solder joints and cleaning the ground connections. I will find out tomorrow if it cranks but doesnt start even with the jump.

When I test drove it, it would stall when I push in the clutch whenever the car is in motion, but if I'm stopped it idles fine. I think the ECM was sort of freaking out because the VSS signal is totally wrong. I will burn the chip with only the VSS corrected and then try again. The big problem is there are no power outlets at which to charge my battery in the entire parking complex, so we have to jump start it each time. Once I get it to run decently I'll just let it idle for a long time to charge the battery up some so we can finish troubleshooting. I think I am facing a combination of problems, so I'm going to do some of the regular maintenance stuff, fuel filter, check all the plugs, etc. There is also a small section of ground wire I still have to check.

What could be causing the lean connection if it turns out my grounds are fine now? The ECM is able to adapt fine, I just don't see why the BLM should be 154 at idle, thats just way too much extra gas with the only major mod to my motor being 13 degree cam retard. Even if I hold the pedal down such that the RPMs go up to 3000 or so with no load, integrator shoots up further and BLM creeps up. Again, injectors have been cleaned and flow tested, the fuel pump is new, and I checked the pressure and it checks out fine at 40psi solid at idle, so no reason to be running lean vs the stock VE tables. I had some vacuum leaks recently but replaced the PCV tubing and connections as well as a cracked vacuum cap. They were all suspect and the car idled much better after fixing them so I don't think I have any leaks left.

-Steven

Shaun41178(2) · Post by **Shaun41178(2)** » Sat Mar 11, 2006 11:38 am

What if your injectors are clogged up? Maybe a good amount of dirt found its way past the filter and got hung up in your injectors?

If Iw as in your position, I wuold replace the injectors.

I actually did have this problem once. I thought it might have been the fuel pump or filter or anythign else. I had a signal from the computer as my test light was flashing. I put different injectors in and it fired right up. I think they might have all been clogged. Hard to imagine I know, but it happened to me.

If the injectors aren't opening even with a good signal, to me, the prob is with the injectors.

Series8217 · Post by **Series8217** » Sat Mar 11, 2006 4:10 pm

Shaun41178(2) wrote:What if your injectors are clogged up? Maybe a good amount of dirt found its way past the filter and got hung up in your injectors?

If Iw as in your position, I wuold replace the injectors.

I actually did have this problem once. I thought it might have been the fuel pump or filter or anythign else. I had a signal from the computer as my test light was flashing. I put different injectors in and it fired right up. I think they might have all been clogged. Hard to imagine I know, but it happened to me.

If the injectors aren't opening even with a good signal, to me, the prob is with the injectors.

That will be a last resort. The injectors only have 4000 miles on them since being cleaned and tested and the fuel filter was replaced a few thousand before that..

Also, would it make any sense that none of the injectors fire at all when they don't fire? They would all have to have an equal amount of dirt.. and they -do- open when the motor is running (higher voltage). Can I test the injectors on the fuel rail by hitting them with a 12V power source? Then if I encounter the no start condition again I could hit them with that and if they open I know whats going on..

w41442 · Post by **w41442** » Sat Mar 11, 2006 4:22 pm

Two problems I see with the 12 volt source. 1, you may exceed the operating current limit of the injectors and fry them if your supply is stronger than your ecm. 2, if they do open, your dumping a lot of fuel down the engine during testing if you dont pull them out. Too much fuel could cause some wear in the cyl wall, and lead to more problems. This is less likely than problem 1.

Series8217 · Post by **Series8217** » Sat Mar 11, 2006 5:02 pm

w41442 wrote:Two problems I see with the 12 volt source. 1, you may exceed the operating current limit of the injectors and fry them if your supply is stronger than your ecm. 2, if they do open, your dumping a lot of fuel down the engine during testing if you dont pull them out. Too much fuel could cause some wear in the cyl wall, and lead to more problems. This is less likely than problem 1.

Would just be a short tap to see that it opens, its easy to hear when it does (plus the fuel pressure would drop). Should I put some resistors in series with the injector before trying this?

Shaun41178(2) · Post by **Shaun41178(2)** » Sat Mar 11, 2006 5:11 pm

I dont' think you will need resistors.

to test to see if my replacement injectors are firing, I did this.

I disconnected all but one of my injectors from the electrical connectors. Then I took the fuel rail off the motor but left it connected to the fuel hoses. I turned the rail up just enough to get a look at the injector and put my finger down by the nozzle. I had a buddy crank it over and no fuel came out. I took a known good injector at time of removal and replaced the injector that didnt' open. Sure enough the injector fired so I knew the other injector was bad, clogged what have you.

Also is it possible that some crud got in the fuel rail after the pressure gauge but won't allow fuel to flow? This will still allow the guage to show good pressure, but yet the injectors are getting little to no fuel. YOu might want to remove the rail and clean it out.

I honestly think its a prob with your injectors and nothing electrical. I read you replaced your computer. Can you put the old one back in and see if this fixes the problem?

whipped · Post by **whipped** » Sat Mar 11, 2006 5:28 pm

How did you wire your injectors? Through one fuse? 2? What are they rated? Can you measure the voltage at the injectors when the engine is running? (Pull one of the injector clips, put the - end of the meter to an engine ground, measure the +12 pin.)

When the engine is cranking is when it's going to be at the lowest voltage. If you've got something acting as a resistance in series with the injectors, then you might not get enough current for them to open. Check to see you're not seeing 10 volts at the injector when the engine is running. If that's the case you need to start checking any and all connections, including things like rusty fuses, bad solder joints, etc.

you can use a 12v source on the injectors to test them. Don't use a car battery in case you slip and end up melting the wire into your hand. I've seen shit like that happen....

Series8217 · Post by **Series8217** » Sun Mar 12, 2006 3:38 am

whipped wrote:When the engine is cranking is when it's going to be at the lowest voltage. If you've got something acting as a resistance in series with the injectors, then you might not get enough current for them to open. Check to see you're not seeing 10 volts at the injector when the engine is running. If that's the case you need to start checking any and all connections, including things like rusty fuses, bad solder joints, etc.

Seems it was a bad ground solder joint or connection. Last night, after cleaning the grounds and replacing some solder joints and a bad crimp (now with a solder joint) it started up fine with a jump start but was still having high BLM and integrator..
..so this morning I replaced the fuel filter and fired it up, then went for a drive. Problem solved!! BLM is 130 with Integrator at 128 at idle and cruising on the highway; where it was 160 before!

whipped · Post by **whipped** » Sun Mar 12, 2006 10:20 am

cool.

Had you continued running that old fuel filter you probably would have been buying a new pump (or engine) shortly. I mean it was restricting what, 20% flow at idle? Ouch.

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Mar 12, 2006 12:43 pm

Well looks like I was wrong. Just glad you found out the problem and all is well now.

SappySE107 · Post by **SappySE107** » Sun Mar 12, 2006 6:53 pm