Fluxuating rpms while idling - Maintenance and Repair Forum

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Fluxuating rpms while idling
Wednesday, August 12, 2020 10:13 AM
My 1990 z24 has been having some idling issues. I've narrowed down the possibilities to a dirty throttle body, vacuum leak,
and a faulty mass airflow sensor. The symptoms closest resemble that of a faulty airflow sensor or a dirty throttle body. The only thing I'm concerned about is being able to find a replacement mass airflow sensor if need be. I've been looking around online for an exact fit with no luck. Anyone got some recommendations for parts websites?

Re: Fluxuating rpms while idling
Wednesday, August 12, 2020 2:01 PM
It doesn't have a mass airflow sensor. Or, did you mean the MAP sensor?
Re: Fluxuating rpms while idling
Tuesday, August 18, 2020 5:24 PM
Don't know if you found the problem yet, but here are a few other things that can cause idle issues.
IAC valve, Stuck EGR, TPS, IAT sensor, ECM engine coolant temperature sensor.

Is your Check Engine light (CEL) on when the engine is running? If it is, pull the code and look it up. It will tell you what component caused the fault.
Here is a video on using the 'paperclip' method.

If the CEL isn't on with the engine running, the best thing I know of is to use TunerPro to see what may be out of specs. You have to have a cable and a laptop (a PC will work, but just isn't as convenient to use).
You can get the cable from Moates; http://www.moates.net/aldu1-and-cabl1-p-127.html?cPath=64
Get TunerPro RT here; https://www.tunerpro.net/downloadApp.htm

This video is a straight forward, easy to understand demonstration of how to get the files you'll need, hook everything up and start data logging.

If you a;ready have a laptop or know someone that can loan you one, that's great. The only cost you'll have is that of the cable. You may find a used cable (and laptop) on ebay, if you look. This will allow you to quickly and easily find most any problem with an OBD1 car. I consider it a must have, if you plan to do your own diagnostics on your car. Heck, you can even help out friends that may have OBD1 cars too. Well worth the small amount of cash you'll spend.

Good luck and let us know what you discovered the problem to be.
Re: Fluxuating rpms while idling
Saturday, August 22, 2020 10:04 PM
My bad, made this thread before bed in a sleep deprived haze and completely forgot about it when I made the other one. I'll definitely give that cable a try. And in regards to your question about my psi on the other thread I believe he said my psi was 35. The check engine light only comes on when idling in park. It comes on after about 2 minutes of idling and the rpms starting going wild. Though putting it in park temporarily fixes the rpms jumping around after its at a stop while in drive.
Re: Fluxuating rpms while idling
Sunday, August 23, 2020 3:16 AM
Okay, first thing to do is pull the code to see what is causing the check engine light to come on. My post above has a link to a YouTube video on how to do it with a paperclip or piece of wire.
Just be sure you connect the paperclip/wire to the correct terminals.

Look closely at the picture and notice the index (circled in red) and note where the A&B terminals are in relation to the index. Make sure your ignition is OFF. Jump the A and B terminals with the wire/paperclip. Turn the ignition to RUN. Do not start the engine. Watch the Check engine light flash. After the first three times it flashes code 12, it will flash the code(s) for whatever is stored in the ECM. Once you get code(s), turn the ignition OFF and remove the wire/paperclip.
Here is a How To that also has the codes to look up.

Let us know what you find.
ALDL Connection.jpg (85k)

Re: Fluxuating rpms while idling
Tuesday, August 25, 2020 12:13 PM
Got around to trying this and it confirmed that it is something with my fuel pressure. The codes were 34 and 45.
Re: Fluxuating rpms while idling
Tuesday, August 25, 2020 5:50 PM
Did you check for vacuum leaks?

I don't have the 1990 service manual.
These are the pages from the 1988 service manual for both code 34 and 45.
NOTE: In box 1 on the right in the code 45, the voltage is 0.75 The . (point) is very hard to see. You probably know it isn't supposed to be 75v, but I wanted to mention it anyway.
You'll need a scan tool, which a Tech 1 is quite expensive, used. I don't know of any cheap ones for OBD1. The cable, laptop and TunerPro are you cheapest way, as far as I know. However, it really is your best way too, as it will do pretty much anything you'd need it for in the future. If you don't yet have a 1990 factory Service manuals, you may want to pick them up. You can normally find them on eBay. If you can get both Book 1 and Book 2, all the better. Book 1 covers service and information, Book 2 is drivability and emissions and electrical diagnosis.

You could just replace the MAP and O2 sensors. Get an; ACDELCO 2131545 GM Original Equipment MAP. Some of the aftermarket ones do not work well.
I don't see a GM O2. Maybe a BOSCH 12014 or a Delphi ES10966 would be okay.

Good luck
Code 34.jpg (670k)
Code 45.jpg (593k)

Re: Fluxuating rpms while idling
Tuesday, August 25, 2020 5:59 PM
I don't think replacement of the sensors will fix it. I already replaced the map sensor last week. I'm thinking I have a bad vacuum leak. I'm going to have a smoke test done to verify.
Re: Fluxuating rpms while idling
Friday, August 28, 2020 9:28 PM
Fixed it with duct tape. Taped up the suspected hose that was causing the leak and it fixed the issue. Time to replace the hose. Thanks for all the info!
Re: Fluxuating rpms while idling
Saturday, August 29, 2020 1:48 AM
Jacob Olson wrote:

Time to replace the hose. Thanks for all the info!
Great! That is an easy and cheap fix. And you're quite welcome.
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