Rough idle with the AC on. Injectors?

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Aug 5, 2021 | 07:07 PM

meshies

Thread Starter

Racer

Joined: Sep 2004

Posts: 259

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From: Louisville KY

Rough idle with the AC on. Injectors?

1992 LT1 ZF6

Hello again C4 family. I decided to use the Air Conditioning on the 92 today. It was running fine all day. Then after sitting for about 1.5 hours, after cranking it up she ran horrible at idle and under load (with ac on) at low rpms. Felt like a misfire. After about 5 minutes it ran fine again.

Now, I'm pretty sure I have a leaky injector. When the car is hot it takes forever to actually fire. It cranks fine but then spins for a few seconds before actually firing. This happens ONLY when hot and has been sitting for a while, when its cold (or just been turned off) it fires up perfect. The car also backfires when decelerating. Today was the first time it actually ran bad, and I am correlating the extra load on the engine from the AC to be the culprit. I might be way off.

Anyway, before I start pulling stuff all apart, what is the best way to pinpoint a leaky injector? Do you guys think a leaky injector might be my problem? I do not have an ALDL reader. I will check for codes using the pin method when I get home.

Aug 5, 2021 | 09:54 PM

Deepa

Burning Brakes

Joined: Mar 2010

Posts: 1,094

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From: Providence RI

Ohm out the injectors and report back. How’s your fuel pressure?

any other signs of misfire?

it’s easy to pull the fuel rails on an lt1. Pull it off, turn on the key so the fuel pump runs, and see if any leak. Be careful of anything that can spark.

Last edited by Deepa; Aug 5, 2021 at 09:55 PM.

Aug 5, 2021 | 10:33 PM

meshies

Thread Starter

Racer

Joined: Sep 2004

Posts: 259

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From: Louisville KY

Quote:

Originally Posted by Deepa

Will ohm testing the injectors report a leak? I just checked the codes, C12 was all that was reported. Do I need to run the car when dealing with engine diagnostics/codes? I just started the car again and it started and ran fine again.

Aug 5, 2021 | 10:43 PM

aklim

Team Owner

Joined: Jul 2004

Posts: 28,501

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From: Hartford WI

Quote:

Originally Posted by meshies

Now, I'm pretty sure I have a leaky injector. When the car is hot it takes forever to actually fire. It cranks fine but then spins for a few seconds before actually firing. This happens ONLY when hot and has been sitting for a while, when its cold (or just been turned off) it fires up perfect. The car also backfires when decelerating. Today was the first time it actually ran bad, and I am correlating the extra load on the engine from the AC to be the culprit. I might be way off...

Your correlation is pretty decent. That said, I don't know if it is leaking or not. Ohm testing is great for if it has failed. Beyond that, it is pretty good for a false positive in that if the thing ohms right, you assume it is good. You can cut off the bottom part of the injector and it still will test good if the coil is good. Only way to really know if the critical things are good is to bench it and see spray pattern and volume. For that, I would send it out. Ohm testing is worthless for leaking injectors so you need to pressurize the system and see if it drops in pressure rapidly after the 2 second prime when you turn the key to "run" position but not crank. After that you need to start crimping off hoses to see if it is before the regulator or after the regulator.

Aug 8, 2021 | 11:48 AM

ctmccloskey

Safety Car

Joined: May 2001

Posts: 4,761

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From: Fairfax Virginia

I would also suggest verifying that your Coolant Temperature Sensor is accurate. When they fail they can cause all sorts of issues like Hard Starting when HOT. You want to verify that the Engine is seeing the real numbers and the only way to do it is to measure what it is seeing. In the FSM there is a chart that shows the resistance Values and their approximate Air temperature it represents.

If it thinks it is cooler then the choke might be trying to kick in, if it is reading hotter than actual it might reduce the amount of fuel which makes it take longer to start. A lot of Hot and Cold starting problems are caused by faulty CTS'.

Last edited by ctmccloskey; Aug 8, 2021 at 11:49 AM.

Aug 9, 2021 | 10:16 PM

aklim

Team Owner

Joined: Jul 2004

Posts: 28,501

Likes: 3,302

From: Hartford WI

Quote:

Originally Posted by ctmccloskey

I would say that the way you mentioned is a way. I would think that you can hit the sensor with an IR Thermometer and verify it against the scan tool.

Aug 11, 2021 | 04:19 PM

meshies

Thread Starter

Racer

Joined: Sep 2004

Posts: 259

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From: Louisville KY

Thanks for all of the replies. I haven't had a chance to get at her yet, probably will this weekend. I will report back with an update when I do.

Aug 13, 2021 | 03:06 PM

meshies

Thread Starter

Racer

Joined: Sep 2004

Posts: 259

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From: Louisville KY

Weekend update.

I ohm tested the injectors, 5 came back 13.5-13.8. 1 came back 10.2, another 10.6 and another 11.2. It seems I have 3 injectors that aren't up to snuff. They probably get worse with heat when the engine is hot.

I tested fuel pressure. Reading came back 41 PSI and dipped to 30 two hours after shut off. PSI stayed regular with engine running and spiked normally when revving. Checked the FPR and that all seems to be fine.