Going Full Lean at Cruise

Rick Simons

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OK, here's something new (to me anyway):
Cruising out to my uncle's place today at 60-65 mph all of a sudden the engine sounded and felt like the fuel was cut, like it does under certain de-cel conditions. The car maintained revs and mph but would not accelerate. Cruise control was off. A/C was off. Gauges displayed normal conditions. Fuel at approx. 75% full. Weather was clear and sunny and about 74F. AFR gauge displayed 22.4 (which I assume is full lean) when this happened. Adding more throttle didn't change anything- car speed and revs remained constant. Lifting the throttle for a few seconds, then resuming the previous (or close to it) throttle setting returned it to normal- AFR gauge fluctuating between approx. 13.4-15.4. It did this twice on the way out to Uncle's house, about a 25 minute drive, and once on the way home. No MIL's, funny noises, or anything else unusual. Tuesday I'm going to repeat this drive cycle and do a data log. Ideas? Thanks!
 

Dino Dino Bambino

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Agree.
If you have an ELM 327 Bluetooth OBD 2 scanner that plugs into your under dash port, and have an app on your smartphone such as Car Scanner that can display and record live data, you'll almost certainly pinpoint the problem.
 

Rick Simons

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Sounds like the TPS to me.
Yeah that's what I was thinking. Wonder if I have enough of a vacuum leak somewhere that the throttle is nearly closed under those conditions and it's telling the PCM that it's decelerating and to cut fuel. Time to do a data log and break out the scanner.
 

Rick Simons

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I'm going to bring Lito in on this too. I was going to asking about tweaking the throttle PID anyway.
 

nfrizell

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Have you changed the fuel filter recently?

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Rick Simons

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I replaced the fuel filter just before I put the tune in it.
I changed the TPS just for the hell of it with a new FoMoCo unit. I drove it for the first time this morning. It still runs pretty much the same, the only difference being it did the max lean thing coming down the driveway @10 mph this morning when cold, then again just now when I left work for an errand. And again, when I let off the throttle all the way then opened it a little it cleared up and went back to reading 13.8-15.2. When it goes wonky it sounds like a v-8 engine that has a carburetor (remember those?) with a jet blocked on one side. Without an MIL and code I'm not sure how to troubleshoot this, other than with a datalog.
 

Rick Simons

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This might prove to be the issue. MAF sensor mounting surface on the JLT intake isn't even close to flat, and it looks like this has been leaking. Think I might put the stock airbox back on and do a revised tune, or figure out a better way to seal this.
MAF_0.jpg MAF_1.jpg
 

Rick Simons

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Forgot to update this thread:
The problem turned out to be the fit between the MAF sensor and the mounting surface of the JLT intake. There was a significant air leak between the two. I took the JLT intake off and put the stock intake on, Lito gave me a revised tune, and it runs perfectly. I've put about 400 miles on it since I put the stock airbox back on, and no problems.
Now to figure out how to fix the MAF pad on the JLT. There are threaded inserts in the plastic that sit up higher than the sealing surface, making it impossible for the MAF sensor to sit flat and seal against its mounting pad on the air duct. To me it looks like a factory/manufacturing defect, and it's probably too late to get a refund or exchange, but I'll check on it.
 

FryDaddy

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Can you cut off or file down the parts that are sticking up?
You can pull the stud out by putting only the screw in pull the stud out put some super glue push it back in below the plastic let sit put all back together.

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Rick Simons

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When I have time to mess with it I'll probably figure out a way to mount it in a mill and make it nice and flat.
 

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