How do you test for a bad injector
Thread Starter
Melting Slicks
Joined: Mar 2008
Posts: 2,141
Likes: 76
From: Santa Maria, CA
How do you test for a bad injector
My 85 has suddenly developed a steady stumble like a miss. I pulled all the plugs and they were good. I have new dist cap, rotor and plug wires. BLMs show it is running rich. All the injectors feel like they are pulsing. The FSM shows an injector balance test but you have to have the special test equipment. Any other ways to test?
Pro
Joined: Dec 2009
Posts: 502
Likes: 0
Yes, anything under 12 ohms is bad and it is best to test after the engine is hot.
Note: A injector stuck open can test good. If you have more than 2 bad injectors and you have 24 lb inj's, contact Jon at FIC, he has great deals on Bosch.
Note: A injector stuck open can test good. If you have more than 2 bad injectors and you have 24 lb inj's, contact Jon at FIC, he has great deals on Bosch.
Thread Starter
Melting Slicks
Joined: Mar 2008
Posts: 2,141
Likes: 76
From: Santa Maria, CA
I have 24# Bosche III from Jon. They ohm at 17.2. They're not that old so now I'm doubting they are the problem.
Le Mans Master
Joined: Jan 2003
Posts: 5,858
Likes: 7
From: Santa Maria CA
Is this a consistent "miss" that follows engine speed throughout RPM or is it random and sporatic?
An ohm test is only part of the story. It can't and won't tell you if any are pissing fuel under pressure.
A noid light on each injector connector will verify that each injector is getting voltage pluse to fire. If you don't have a noid light, a mechanic's stethoscope on each injector is more accurate than going by feel. You can hear these injectors firing very loudly with one of these it's unreal.
One home test is by raising the fuel rail and suspending each injector in a jar while under pressure to allow for a visual check of leaks. I would only do this if you find excess fuel pressure drop with a fuel pressure guage and you've isolated the fuel rail by pinching off the return and/or feed lines under the rubber fill matt out back as outlined in the FSM. If your pressure holds within spec you know the injectors can't be leaking fuel.
Have you checked for vacuum leaks? Unmetered air entering the motor will throw off the mixture and can cause the ECM to overcorrect. Spraying carb cleaner around hoses and gasket joints while listening for the idle to change works quite well.
Are you going into closed loop?
An ohm test is only part of the story. It can't and won't tell you if any are pissing fuel under pressure.
A noid light on each injector connector will verify that each injector is getting voltage pluse to fire. If you don't have a noid light, a mechanic's stethoscope on each injector is more accurate than going by feel. You can hear these injectors firing very loudly with one of these it's unreal.
One home test is by raising the fuel rail and suspending each injector in a jar while under pressure to allow for a visual check of leaks. I would only do this if you find excess fuel pressure drop with a fuel pressure guage and you've isolated the fuel rail by pinching off the return and/or feed lines under the rubber fill matt out back as outlined in the FSM. If your pressure holds within spec you know the injectors can't be leaking fuel.
Have you checked for vacuum leaks? Unmetered air entering the motor will throw off the mixture and can cause the ECM to overcorrect. Spraying carb cleaner around hoses and gasket joints while listening for the idle to change works quite well.
Are you going into closed loop?
Last edited by 86PACER; May 17, 2010 at 07:23 AM.
Thread Starter
Melting Slicks
Joined: Mar 2008
Posts: 2,141
Likes: 76
From: Santa Maria, CA
Is this a consistent "miss" that follows engine speed throughout RPM or is it random and sporatic?
An ohm test is only part of the story. It can't and won't tell you if any are pissing fuel under pressure.
A noid light on each injector connector will verify that each injector is getting voltage pluse to fire. If you don't have a noid light, a mechanic's stethoscope on each injector is more accurate than going by feel. You can hear these injectors firing very loudly with one of these it's unreal.
One home test is by raising the fuel rail and suspending each injector in a jar while under pressure to allow for a visual check of leaks. I would only do this if you find excess fuel pressure drop with a fuel pressure guage and you've isolated the fuel rail by pinching off the return and/or feed lines under the rubber fill matt out back as outlined in the FSM. If your pressure holds within spec you know the injectors can't be leaking fuel.
Have you checked for vacuum leaks? Unmetered air entering the motor will throw off the mixture and can cause the ECM to overcorrect. Spraying carb cleaner around hoses and gasket joints while listening for the idle to change works quite well.
Are you going into closed loop?
An ohm test is only part of the story. It can't and won't tell you if any are pissing fuel under pressure.
A noid light on each injector connector will verify that each injector is getting voltage pluse to fire. If you don't have a noid light, a mechanic's stethoscope on each injector is more accurate than going by feel. You can hear these injectors firing very loudly with one of these it's unreal.
One home test is by raising the fuel rail and suspending each injector in a jar while under pressure to allow for a visual check of leaks. I would only do this if you find excess fuel pressure drop with a fuel pressure guage and you've isolated the fuel rail by pinching off the return and/or feed lines under the rubber fill matt out back as outlined in the FSM. If your pressure holds within spec you know the injectors can't be leaking fuel.
Have you checked for vacuum leaks? Unmetered air entering the motor will throw off the mixture and can cause the ECM to overcorrect. Spraying carb cleaner around hoses and gasket joints while listening for the idle to change works quite well.
Are you going into closed loop?
Yes it is a consistent miss. My first thought was a plug wire, but it doesn't feel like a completely dead cylinder. The injector harness is good. I can feel all the injectors pulsing.
I've checked all the vacuum lines. Leaks will usually increase RPM. The RPM is normal. I think I've replaced all the hoses with new rubber so there shouldn't be any cracks.
I can't tell if I have a leaky injector. I replaced the fuel pump about a year ago and reinstalled the pulsator rather than a hose. So my pressure always drops after a few minutes. Its one of those jobs that has been on the to-do-list. When I pulled the spark plugs I did not see any that were wet or fouled.
Yes the computer is going into closed loop. It takes longer than it should because I'm running headers with the O2 on the collector.
I looked at a scan file from when the engine was running good and compared to a scan now. They are nearly identical. Tells me whatever is going on must be on the bank that doesn't have an O2 sensor. O2 sensor voltage averages .35v to .8v.
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