1997 5vz-fe weird misfire only at idle!!

jharri7140 · Jun 3, 2022

Running into an issue recently of my 4runner misfiring at idle. A couple months ago it developed an issue where on cold starts after a night of sitting it would fire up and run rough for about ten seconds then it would go away and be ok. Then recently it is still doing it on cold start, but now when it gets to running temp and I come to a complete stop and idle it runs super rough, and even threw a misfire code. Also a couple months ago I had a misfire while driving on the highway so I replaced plugs, wires, and coils. Then in an effort to make sure fueling was good I rebuilt the fuel injectors last weekend. So now I know I have solid spark and fueling, so now I am thinking some sort of a vacuum leak or intake valve. Does anyone know what this issue is, or have any advice what to check or clean?

octanejunkie · Jun 3, 2022

You didn't give.us much info about the rig, only what you've done.

Check for vacuum leaks, IAC, TPS and air filter/MAF

Welcome to the forum!

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Bad Luck · Jun 3, 2022

octanejunkie said:
You didn't give.us much info about the rig, only what you've done.

Check for vacuum leaks, IAC, TPS and air filter/MAF

Welcome to the forum!

Sent from my Pixel 4 XL using Tapatalk

I'm just glad he finally hit his 15 post minimum

jharri7140 · Jun 3, 2022

Its a 1997 2wd limited with 275k miles. Everything is stock except for a couple things hear and there like air lift bags.

brillo_76 · Jun 3, 2022

Here is the coil spark plug information. So you can rule out those parts being the cause:

Spark Plugs Wires and Coil Information for the 5VZ-FE

jharri7140 · Jun 14, 2022

Thanks for the information on checking the coils, I gave a new IAC a try and now the problem is still there but not as noticeable as the thing is idling at 1500 rpm. And while I was in there I cleaned the MAF sensor and checked vacuum lines and the throttle body fly, everything looks to be ok. I will check out the coil resistances to ensure they are good, it would just surprise me the misfire would only be happening at idle.

jharri7140 · Jun 14, 2022

My thought due to it being an only idle misfire is it is a vacuum leak and not an ignition problem. Is there a good way to check for vacuum leaks?

Bad Luck · Jun 15, 2022

jharri7140 said:
My thought due to it being an only idle misfire is it is a vacuum leak and not an ignition problem. Is there a good way to check for vacuum leaks?

Warm engine idle should be around 700 rpm in park/neutral. To check for vacuum leaks spray suspect areas with carb cleaner with the engine running and listen for rpm changes.

jharri7140 · Jun 15, 2022

Bad Luck said:
Warm engine idle should be around 700 rpm in park/neutral. To check for vacuum leaks spray suspect areas with carb cleaner with the engine running and listen for rpm changes.

Yeah I am under the impression the high idle is from the computer having not learned the new IAC. I will try out spraying carb cleaner and see if I can figure it out.

Bad Luck · Jun 15, 2022

jharri7140 said:
Yeah I am under the impression the high idle is from the computer having not learned the new IAC. I will try out spraying carb cleaner and see if I can figure it out.

What are your long term fuel trims at idle once the engine is warm?

BlackWorksInc · Jun 19, 2022

jharri7140 said:
Running into an issue recently of my 4runner misfiring at idle. A couple months ago it developed an issue where on cold starts after a night of sitting it would fire up and run rough for about ten seconds then it would go away and be ok. Then recently it is still doing it on cold start, but now when it gets to running temp and I come to a complete stop and idle it runs super rough, and even threw a misfire code. Also a couple months ago I had a misfire while driving on the highway so I replaced plugs, wires, and coils. Then in an effort to make sure fueling was good I rebuilt the fuel injectors last weekend. So now I know I have solid spark and fueling, so now I am thinking some sort of a vacuum leak or intake valve. Does anyone know what this issue is, or have any advice what to check or clean?

What do you mean you rebuilt the fuel injectors?

Also when you have this issue on cold-start have you noticed more smoke/vapor from the tailpipe? Have you done a compression test and/or leak down test on the vehicle while you were changing plugs and such? What was the misfire code(s)?

jharri7140 said:
Thanks for the information on checking the coils, I gave a new IAC a try and now the problem is still there but not as noticeable as the thing is idling at 1500 rpm. And while I was in there I cleaned the MAF sensor and checked vacuum lines and the throttle body fly, everything looks to be ok. I will check out the coil resistances to ensure they are good, it would just surprise me the misfire would only be happening at idle.

Something I ran into with an older Tacoma that had the cable driven throttle body was that the center shaft for the throttle body had actually worn out enough to "stick" around the idle position. It would act like the dash pot was holding it open (you could physically press the linkage fully closed and it would idle fine) but if you revved the vehicle it would stick and idle high at about 1500+RPM. The fix was a new throttle body unfortunately. I didn't end up keeping that part because when I inspected it after replacing I could see where the throttle plate was catching on the bore and how much play was in the shaft, so I didn't think it would be worth rebuilding IF it was even possible to do so.

Bad Luck said:
What are your long term fuel trims at idle once the engine is warm?

This is also something worth looking at, this will give us a better idea of how the engine is running from a fueling/efficiency standpoint and help narrow down possible culprits.

jharri7140 · Jun 20, 2022

BlackWorksInc said:
What do you mean you rebuilt the fuel injectors?

Also when you have this issue on cold-start have you noticed more smoke/vapor from the tailpipe? Have you done a compression test and/or leak down test on the vehicle while you were changing plugs and such? What was the misfire code(s)?

Something I ran into with an older Tacoma that had the cable driven throttle body was that the center shaft for the throttle body had actually worn out enough to "stick" around the idle position. It would act like the dash pot was holding it open (you could physically press the linkage fully closed and it would idle fine) but if you revved the vehicle it would stick and idle high at about 1500+RPM. The fix was a new throttle body unfortunately. I didn't end up keeping that part because when I inspected it after replacing I could see where the throttle plate was catching on the bore and how much play was in the shaft, so I didn't think it would be worth rebuilding IF it was even possible to do so.

This is also something worth looking at, this will give us a better idea of how the engine is running from a fueling/efficiency standpoint and help narrow down possible culprits.

Yes when this problem initially began I noticed on cold start I have vapor coming out of the exhaust pipe, also the engine began losing coolant but not on the ground below.

jharri7140 · Jun 20, 2022

Also I made sure to check the throttle body for the issue of not closing all the way as I had seen it before on another thread. It closes completely so the issue is not there. I also checked for vacuum leaks using brake cleaner, and there were no vacuum leaks causing the idle to fluctuate. The conclusion seems to be coming down to the cold start exhaust vapor and unseen coolant leak.

brillo_76 · Jun 20, 2022

What does your compression checks show? As you may have a seeping head gasket. A coolant leak down test is needed as well to verify all is well.

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jharri7140 · Jun 20, 2022

brillo_76 said:
What does your compression checks show? As you may have a seeping head gasket. A coolant leak down test is needed as well to verify all is well.

Sent from my SM-A505U using Tapatalk

I will have to get around to compression testing it, interesting thing is the oil has no signs of water in it.

brillo_76 · Jun 20, 2022

jharri7140 said:
I will have to get around to compression testing it, interesting thing is the oil has no signs of water in it.

Could just be a microscopic mist and is burning up.. You can see that in the spark plugs mostly too..

Bad Luck · Jun 20, 2022

jharri7140 said:
I will have to get around to compression testing it, interesting thing is the oil has no signs of water in it.

You would only see coolant in the oil if the head cracked or the head gasket failed between a coolant passageway and oil passageway. Head gaskets typically fail between the cylinder bore and coolant passageway or between cylinder bores.

jharri7140 · Jun 21, 2022

Bad Luck said:
You would only see coolant in the oil if the head cracked or the head gasket failed between a coolant passageway and oil passageway. Head gaskets typically fail between the cylinder bore and coolant passageway or between cylinder bores.

Yeah my conclusion was it is just leaking strictly into the cylinder. Judging by how much coolant I am losing my thought is it is a fairly small leak. The effects I am seeing from it is sometimes causing a cylinder misfire at idle, as well as a slight decrease in mpg. So for now I am going to just let it be and watch the coolant levels.

Bad Luck · Jun 21, 2022

jharri7140 said:
Yeah my conclusion was it is just leaking strictly into the cylinder. Judging by how much coolant I am losing my thought is it is a fairly small leak. The effects I am seeing from it is sometimes causing a cylinder misfire at idle, as well as a slight decrease in mpg. So for now I am going to just let it be and watch the coolant levels.

If you let it like that for too long it can corrode the block and then you will need to have the engine resurfaced or get a new engine.

octanejunkie · Jun 21, 2022

jharri7140 said:
Yeah my conclusion was it is just leaking strictly into the cylinder. Judging by how much coolant I am losing my thought is it is a fairly small leak. The effects I am seeing from it is sometimes causing a cylinder misfire at idle, as well as a slight decrease in mpg. So for now I am going to just let it be and watch the coolant levels.

Bad Luck said:
If you let it like that for too long it can corrode the block and then you will need to have the engine resurfaced or get a new engine.

Blue Devil?

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1997 5vz-fe weird misfire only at idle!!

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