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Lean Idle, Breaking Up - Boost Leak?

 
Old Mar 25, 2019, 10:46 AM
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Question Lean Idle, Breaking Up - Boost Leak?

Hey guys, new here to EvoM and the Evo/DSM world. Just bought an '03 BBY Evo, new engine and stage one shep trans. Very light mods, recent tune on 93 oct (did full inspection/fixed timing, boost leaks, etc by tuner), only making ~320Hp. Was running absolutely spectacular for awhile, now has suddenly started to have weird idling problems. On cold starts in the morning - fires up fine, normal AFR ~15 for a few seconds, then leans out ~18+ AFR and runs slightly rough, like the timing is off or its breaking up. After it warms up a bit, if I tap the gas it will bounce back to normal AFR's and run normal. When leaving work later in the day - it always dies on the first start, then runs lean and idles rough for a bit then returns to normal. Some research tells me that this is a boost leak - haven't noticed any significant loss in power or spool. I just want to confirm that I am in the right direction and see if anyone has a better idea of what it could be. What are some common spots to have a boost leak on Evo's? Have given all the vacuum lines, couplers, intercooler/piping and throttle body a once over, with no indications of a leak. Plan on doing a boost leak test this coming weekend.​​​​​​​



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Old Mar 25, 2019, 12:20 PM
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When my Evo had an odd idle like yours, it ended up being a small exhaust leak at the down pipe and cat connector. It was a small enough leak that the only time you could hear it is when you poked your head under the car. I honestly didnt think an exhaust leak of that size would have caused the AFR to jump around so much but after replacing the gasket, problem solved.
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Old Mar 25, 2019, 12:51 PM
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Originally Posted by Lumpy Sticks View Post
When my Evo had an odd idle like yours, it ended up being a small exhaust leak at the down pipe and cat connector. It was a small enough leak that the only time you could hear it is when you poked your head under the car. I honestly didnt think an exhaust leak of that size would have caused the AFR to jump around so much but after replacing the gasket, problem solved.
I'll definitely take a look at the exhaust, thanks!!
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Old Mar 28, 2019, 11:11 AM
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No exhaust leaks present, still have yet to make a boost leak tester and give that a shot this weekend. It was holding low amounts of boost decently on the way home from work last night, so I'm starting to be skeptical if it could be that either. Going to try downloading the tuning software for speed density (TephraMod??) and making a log of a cold start to see where the problem is coming from - ignition, fuel or air. I'm used to using Hondata so this is all a one off for me. Any help is greatly appreciated!!
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Old Mar 30, 2019, 08:44 AM
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Boost leak test confirmed there was multiple leaks. A capped off vacuum line on the intake manifold had cracked and was leaking, the TPS has 1 bolt and RTV holding it down... pretty large leak there, and a plate located on the back of the TB under the TPS needs to be RTV'd again, also a large leak. So this leads me to ask - Where is everyone buying their TPS? I know someone in the area that parts Evo's, I can get one for $70 from him. Or i can buy from the dealer new, which I can only assume will cost me my left nut.. I also found some cheapy $15 TPS on Ebay/Rock Auto... worth the shot?? Can also get what looks like OEM replacement from Rock Auto $80 new. Right now I'm thinking I'll just buy used from the guy that parts Evo's - I always try to go with OEM Quality.
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Old Apr 1, 2019, 10:09 PM
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I donít know if this might help,
but on my evo 6, I installed a new bov and it vented to atmosphere.
This ended up causing my car to stall and created odd idling problems. Changed back to stock bov and itís sweet.
But i donít know what type of setup you got
could be a leak somewhere
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Old Apr 2, 2019, 06:59 AM
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Originally Posted by Evo6666 View Post
I don’t know if this might help,
but on my evo 6, I installed a new bov and it vented to atmosphere.
This ended up causing my car to stall and created odd idling problems. Changed back to stock bov and it’s sweet.
But i don’t know what type of setup you got
could be a leak somewhere
Thanks for the suggestion =) I haven't changed anything mechanical since purchasing the car, and I found the culprit. Biss screw O-ring had worn and was causing a large leak, and the "Plate" I mentioned in my last post was actually the flapper valve (Only on '03/'04 Throttle Bodies), which had an enormous leak. The O-ring on that flapper valve had basically disintegrated. Replaced O-rings in both leaky spots. The O-ring I put on the Biss screw actually made the leak worse and thus the symptoms I was getting had gotten worse. I have a Biss screw/O ring on the way, will check back after thats installed. Boost Leak tester is the best $10 I think I've ever spent!
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Old Apr 3, 2019, 07:01 AM
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Boost leak tests are key, glad you found the leaks. Other common leak points are:

- one of the EGR vacuum lines underneath/behind the intake manifold tends to pop off. It's not held on to the nipple with a clamp or anything from the factory, so it's a good idea to zip tie it on there (I zip tied all the vacuum lines that see boost).
- throttle body shaft seals (Mil.Spec sells upgraded seals).
- fuel injector seals

Last edited by nirvancol; Apr 3, 2019 at 07:08 AM.
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Old Apr 7, 2019, 10:57 AM
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The new Biss Screw and O ring are in, and unfortunately didnt fully fix the problem... Driveability is better and it seems to run a touch better, but I'm still getting a jumpy/rough and lean idle. After installing the screw I took it out for a quick spin to test it out. I did a pull and my AFR's were 10 basically the whole pull. Brought it back to the garage, did another boost leak test, no hissing at all, slowly losing pressure about 1psi per second, which seems normal from my research. Sprayed the items mentioned above with soapy water and couldn't locate any type of leak. However, I found that my oil fill cap has a chip in it, breaking off one of the tabs. Before I did a pull I noticed there was oil on top of my valve cover and consequently on the oil fill neck - it was obvious the tiniest amount of oil was seeping from it, but could this really be the source of my problems? Seems like wishful thinking... I'm going to replace it immediately anyways, but i kind of doubt that could be the source issues.
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Old Apr 7, 2019, 11:14 AM
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If you have no more boost leaks, you could check the usual culprits: O2 sensor, fuel pump, MAF, etc.

Brainstorming, not necessarily in order LOL:
1. The fact that it has normal AFR in open loop, then lean in closed would "normally" indicate it's a sensor.
2. Assuming the tune hasn't been changed and you are now seeing 10:1 under WOT would support a boost leak theory or a MAF issue. Are you testing at least up to the PSI the car sees? And yes, dropping 1psi per second is acceptable, from what I've been told. When my car is buttoned up, it usually drops slower. Assuming you are boosting up to 20-25psi, that is.
3. When the car starts idling lean, does it stay there? Does it eventually fix itself? Does it go back and forth?
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Old Apr 8, 2019, 07:19 AM
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Thanks for your help! I forgot to mention that it is converted to speed density. I'm testing the car at 15-20psi, it's tuned for 23psi. I will turn the car on and it will have a normal idle/afr for 10-30 seconds and then start running rough and lean out until the car warms up, most of the time it will stay lean until i tap the gas and then it will correct itself and return to normal afr. Once its warmed up the idle will be smooth but afr's will remain lean until i tap the gas. I realize this corresponds with what you're saying about the open loop vs closed loop, but I'm convinced there is some kind of boost leak I'm missing. It makes sense if you think about it - lean under vacuum and rich under boost means there's a leak somewhere in the system. This coming weekend, I'm going to bring it to a friends and take off the front bumper, spray down the entire system with soapy water and boost leak test it again. I'll be taking a close look at the PCV system, intercooler, charge pipes, injector seals, throttle body and intake manifold. I printed off an OEM vacuum line routing diagram and will be going off of that. I also haven't mentioned that I have an EGR delete. If I fail to find any boost leaks then I'll move onto looking at the 02 sensor, fuel pump, etc that you mentioned and it probably wouldn't hurt to hook up to a laptop and make a log of what's going on.

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Old Apr 8, 2019, 11:17 AM
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Yeah, A data log of the MAF may help confirm a vacuum/boost leak.
Either way, good luck. Hopefully you get out sorted asap
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Old Apr 15, 2019, 12:06 PM
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During the tests this weekend I found more boost leaks - leaky plastic splitter for my boost gauge was replaced with brass fitting (It's split off of the FPR... ) , Blox manual boost controller was leaking, tightened the fitting (there's also a pin hole on the fitting... looks like it was manufactured that way... vented manual boost controller??). I'm fairly confident there isn't any more boost leaks at this point, I sprayed down everything in the system with soapy water, made sure everything was zip-tied and secure. No hissing, and holds pressure in the system a lot better. Going to download Mitsulogger and make some logs. I have what looks like a blue VGA cable that comes out of the ECU, do I need the Tactrix cable on top of that to use logging/tuning software?
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Old Apr 15, 2019, 01:23 PM
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Yeah, MBCs have holes, that's how they bleed pressure. I'm not sure what kind of cable attaches directly to the ECU. If it's VGA, then it's probably something outdated unless it's actually coming from a wideband sensor. That has me very curious. You need a Tactrix cable to log with EvoScan and flash/read with ECUFlash. Mitsulogger as well, IIRC.
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Old Apr 15, 2019, 02:07 PM
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Originally Posted by kaj View Post
Yeah, MBCs have holes, that's how they bleed pressure. I'm not sure what kind of cable attaches directly to the ECU. If it's VGA, then it's probably something outdated unless it's actually coming from a wideband sensor. That has me very curious. You need a Tactrix cable to log with EvoScan and flash/read with ECUFlash. Mitsulogger as well, IIRC.
Ran out to my car quick to verify, and I think you're right... Previous Owner told me this was the connection to the ECU.. I followed the plug and it runs under the trim piece of the shifter.




While I was out there I took pics of my broken oil fill cap. You can see where its seeping out onto my valve cover. Must be getting a good amount of crankcase pressure/blow-by.....



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