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P0121 during deceleration

Agreen

NAXJA Forum User
Location
Charleston
I see that the P0121 is a common issue with the 4.0, and often erroneously replaced. I hate replacing parts without diagnosis first, so I will not just throw a TPS at it in hopes of fixing it. I have an ELM327 Bluetooth scanner that shows it at 12.5% key on/engine off and at idle. At wide open it reads about 88%. Voltmeter test confirms about 0.6v and 4.5v, smooth sweeping motion between. I have tested this at the sensor and at the ECM. The wiring and TPS checks out, even with a wiggle test of all wiring.

I have looked more into it and found that it's an issue of the TPS not matching expected position based on MAP signal. This problem *usually* happens when I downshift to maintain speed going down a hill, with no throttle.

I noticed also that my AC went from recirc to defrost while going uphill. I haven't been able to locate a vacuum leak (yet) but I am suspicious of the manifold gasket. I replaced the exhaust manifold with a dorman unit like 2 years ago, and I've just started getting another exhaust leak. I read that the dorman gasket is kind of trash, but unfortunately after I had already installed it.

So I'm thinking this weekend I'll be ripping off the manifold again, inspecting for cracks, and replacing the gasket at a minimum.

Has anyone else experienced a P0121 because of a manifold leak? I'm going to document this as I go so hopefully others don't waste their time and money throwing TPS's at their own.
 
You didn't say what yr, but that would suggest a bad connection/wiring.
 
You didn't say what yr, but that would suggest a bad connection/wiring.


That 0.6-volts closed seems a bit high. Check the ground pin on the TPS and make sure it has a good ground. Also try cleaning the throttle body in-case it's just crude preventing the throttle plate from fully closing.
 
Thanks for the replies. It's a 98, but that really doesn't matter, as they're all pretty much the same after 96 except for the different ignition. The wiring has been gone through with a fine toothed comb. I rounded up to 0.6 for the sake of the discussion, but it was about 0.55.

There's really nothing wrong with the TPS as far as I could tell. Also, not sure what you mean by vacuum line @ TPS (laswsoncl). The TPS doesn't have a vacuum line.

I pulled the manifolds to find that there was definitely a leak. The gasket was burned through, and badly. #2 exhaust port was blown out, and was causing a vacuum leak at #2 intake port. #2 and #3 exhaust runners were cracked 360 degrees where it met the #1 runner.

Remember, this is a Dorman exhaust manifold and gasket that was changed about 2 years ago. I welded the cracks, but I fully expect them to return. As of now, there have been no further codes, and I was quite surprised at the significant increase in power. What also troubles me is that I never got a misfire code, and never had a high or rough idle from the vacuum leak.

Anyway, I guess time will tell. At least I've solved my downhill CEL though... for now.
 
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