High FT diagnostics with sprays

[afpj]

Hey team,

02 Yukon 5.3 FF 216K miles

Trying to chase down what I believe is an intake leak somewhere, high LT fuel trims 25% both sides at idle , goes to normal with throttle input. STFTs are essentially normal at idle or throttle. I think this Is Classic evidence of intake leak, LTFTs exactly the same on both sides though. Runs fine.. so in trying to diagnose the source, using various flammable sprays, I've tried MAF cleaner cuz it says it's flammable- didn't get anywhere, tried electronics cleaner cuz it also said flammable but didn't find anything. The usual spray I read about is carb cleaner but it says not good for plastics and well, our intake is completely plastic. It's CRC carb and throttle body cleaner in particular. Curious what everyone's opinion about that spray for trying to diagnose high fuel trims but yet not mucking up the plastics? I even tried some butane from a little butane torch used for cooking, like for putting a crust on a creme brulee, but didn't find any changes in RPM similar to the previous sprays. As always, your opinions are much appreciated.

Oh, and no codes, not even pending codes

 

[Scottydoggs]

are the trims -25 or +25. 25 is the max most scanners will read.

neg lt trim is rich. pos is lean. incase theres any confusion with that part. as its backwards lol

 

[afpj]

Oh duh, +25 sorry.

 

[Scottydoggs]

ok, that should be setting a lean code. being it dont have one it should soon.

if your really dont want to use carb cleaner you can try propane. follow your black plastic vac lines too. they tend to break when coming up on 20 years old. that line up top the manifold to the pcv too. brake booster hose if you have a vacuum booster. that grommet tends to dry rot, and boosters tend to rot out too. but you'd be here talking about a rock hard brake pedal too then lol

 

[afpj]

I forgot about the brake boost line, at the booster itself, will have to check that. For the plastic vac lines, will the one that goes to the purge solenoid set a evap leak code if that was leaking somewhere? I sprayed that one but only chased it to the back of the intake.
Still curious if anyone had used the carb throttle body cleaner and not experience any plastic part deformities that is implied on the spray can warning (Avoid paint and plastics)

thanks!

 

[nonickatall]

That's no problem at all, if you use this spray to detect a vacuum leak and spray it into the throttle valve at the front, the gas is so fast in the engine, that it can't affect the plastic parts at all.

If you clean the plastic parts with it, spray it and rub it with a rag, that's a problem. But apart from that, the search for vakuum leak with these sprays usually doesn't work well, because if you have a small leak, you hardly increase the idle speed. You need huge leak to have a relevant idle increase. It is better to do a smoke test. So blow smoke at the front via the throttle valve into the engine and see if it smokes out somewhere. You can also do this yourself with simple tool, means by taking a cigarette and converting an air pump accordingly. There are plenty of instructions on the internet for this.

But if your engine runs to lean, it must not be a intake leak, it can also be a problem with the mass air sensor, an oxygen sensor, even with a clogged cat.

 

[MassHoe04]

I had one vehicle with high fuel trim issue. Exhaust leak at the flange where the collector pipe clamps on.

The gap between the flange and collector was pulling excess air in by Venturi effect.
Excess air was detected by the upstream O2 sensor as a lean condition requiring more fuel in attempt to balance out to where the computer expects it to be.
The computer tired its best to compensate and made injectors work overtime pumping the maximum flow of gas.

Ran super rich! You could smell unburnt fuel at the exhaust!

I loosened the clamp, re-seated the collector pipe (a few love taps wit a BFH on the flare of the collector to make it mate with the flange nice and flat all around), put a jack under the collector to hold it in place tight, then tightened the clamp up real good.

No excess air. O2 sensor no longer thought there was a lean condition and fuel flow at the injectors went back to normal.

Ran fine ever since.

May not be your issue, but this is what I had happen to me one time. Maybe, it will help. Won't hurt to look/listen for exhaust leaks above the upstream O2 sensor... Free anyway.

 

[afpj]

Thanks for suggestions. I did replace the MAF with an OEM one from RockAuto, it was a Delphi just like the original, also replace the upstream O2 sensors which had about 100K miles on them, just for preventative maintenance, used denso. This was after the fuel trims already started being wonky so it's been a few months.
, I'm trying to figure a homemade smoker using mineral oil...getting a steady low pressure flow with a compressor is where I'm stuck. Will tackle after my shift in a few days.
I thought about the exhaust flange...I don't hear hissing, and it's both sides simultaneously, so I thought what are the chances that both are doing it same time? Will have to investigate further... I did check to see if the nuts were tight on those studs where the exhaust manifold attaches to the pipe, they're on solid but it could be because of age etc. Thank you!

 

[mattbta]

I had one vehicle with high fuel trim issue. Exhaust leak at the flange where the collector pipe clamps on.

The gap between the flange and collector was pulling excess air in by Venturi effect.
Excess air was detected by the upstream O2 sensor as a lean condition requiring more fuel in attempt to balance out to where the computer expects it to be.
The computer tired its best to compensate and made injectors work overtime pumping the maximum flow of gas.

Ran super rich! You could smell unburnt fuel at the exhaust!

I loosened the clamp, re-seated the collector pipe (a few love taps wit a BFH on the flare of the collector to make it mate with the flange nice and flat all around), put a jack under the collector to hold it in place tight, then tightened the clamp up real good.

No excess air. O2 sensor no longer thought there was a lean condition and fuel flow at the injectors went back to normal.

Ran fine ever since.

May not be your issue, but this is what I had happen to me one time. Maybe, it will help. Won't hurt to look/listen for exhaust leaks above the upstream O2 sensor... Free anyway.

I'm going to have to check this out. Mine runs super rich and I know for a fact I have an exhaust leak at the rear of the y-pipe after having my RMS done. I suppose I could have a leak at the manifold flange, too, since the whole thing came out to pull the transmission. Usually -10 to -20 LTFT on both banks. I just haven't gotten under it to mess with it...lazy as posted in the daily thread.

 

[afpj]

So in chasing down what @MassHoe04 mentioned, I looked at my exhaust manifold to y-pipe connection and noticed only the downstream side has the nut. Is that normal? The upstream side only has the bolt sticking through. I thought there were nuts on both sides? All three studs look like this, on both sides.

 

[MassHoe04]

Appear to be threaded studs for these. Threaded into the top flange. No nut on top. Nut on bottom only.

Still an area to check out. Make sure nothing is loose or has any gaps there.

 

[MassHoe04]

Maybe exhaust flange connections are less likely to happen on these. Not sure...

The two parts on the GM are flat. Probably less likely to form gaps that leak, unless the flange warped.

Where on my other vehicle the cast part of the flange was a tapered cone shape where the collector pipe mated up. It had a two-piece ring that clamped the two sides together. If the pipe and the flange were not mated up straight or the flare on the pipe did not fit the cone shape on the flange perfectly, you get a leak.

Was just something I thought would be easy to rule out and cost nothing to find out.

 

[afpj]

Well, I've been silent on this topic because summer came and the problem went away. Fuel trims back to normal. Then winter came last month and fuel trims went up again. So cold weather makes it look like I have a vacuum leak. I used a nice flammable spray like throttle body cleaner when it was cold (45 is cold here) and sure enough RPM's bumped up if sprayed near the intake manifold gaskets. I've never done a job like this and only because of reading all the great advice on this forum and watching a couple of YouTube videos, I was able to tackle the job. A few minor hiccups like, snapping the white heater hose T (although as you can see in the picture, it wasn't really white anymore. It was tan to brown so it look like it was going to go anytime anyway- God works in mysterious ways). After that though, I got really discouraged and just wanted to button it up so I forgot to put on the RTV dam around the knock sensors, also forgot to put loctite on the intake bolts. It looks like the shop that did it previously did not use loctite and it lasted for a good 100k miles. So I guess I'm okay on that. Fuel trims back to normal even in the cold weather.

Thanks to this awesome forum... happy New Year!