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High positive fuel trims, low maf grams at idle

Old 01-03-2014, 12:18 PM
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High positive fuel trims, low maf grams at idle

I know the title seems to describe a vacuum leak. I have been all over that engine and every vacuum line I can find with carb cleaner and propane but cannot get any idle changes. I have even tried jiggling hoses and the intake tube while I am spraying the carb cleaner or propane. I have even tried spraying all around the engine at any point where a seal might be like the UIM. Made sure my sparkplugs were still tight as suggested by someone else.

The mods done to the car are midpipe and exhaust. I have a new air filter since yesterday and also removed the first MAF screen because it had a rather large hole in it. That smoothed out my maf readings but they are still low. Spark plugs are fairly recent less than 5k miles I think. The coils I replaced at 35k miles with OEM and it is at 51k now. The MAf itself is less than 10k miles old. I replaced it before with a denso unit thinking that may have been the problem but nothing changed. I cleaned it yesterday but it was spotless already.

My MAF is getting 4 g/s at idle and I cannot hit more than 207 g/s at WOT. I did a compression test last night and one rotor got 107 104 104 at 298 rpm the other got 104 107 104 at 288 rpm. I used the foxed.ca calculator to account for altitude and the renesis correction factor. I just averaged the rpm to 293 to make it easier qwhich puts the numbers at 107 110 107 @ 250 rpm sea level which according to the charts on here is still decent. I'm not sure if the lower compression than new would cause me to lose 1 g/s at idle or not.

My LTFT has been very high for a while now at 15+ and my STFT I have seen at 8+ many times while just idling but is usually around 3+. This is a log after I did the compression test and reset my trims by just pulling the negative battery wire off for a while. It is in CSV format. I started out going slow and then got on the highway and set cruise control to 65 in 6th gear. I went down maybe 6 exits and turned around and gave it arount half throttle to near redline in a few gears when getting back on the highway and set cruise to 65 again this time in 5th gear. Once I got off at my exit I did a quick WOT in first. I am going to keep logging as the computer learns again and see if it gets worse still.

Anyone have any idea what could be causing my high fuel trim? Also like I stated above, could my compression be the culprit for the MAF 4 g/s at idle and not being able to get over 207 g/s at WOT? It is usually more like 200 g/s at WOT but a few times it has been higher.

Last edited by xexok; 01-03-2014 at 12:28 PM.
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Old 01-03-2014, 12:50 PM
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The g/s at WOT can vary considerably from one MAF to the next, so I wouldn't put much stock in that. It doesn't sound abnormal though, offhand.

4g/s at idle with high positive fuel trims means that ECU is seeing lower than normal air past the MAF, but then it's seeing it far too lean at the O2 sensor, so it's dumping in fuel. It/s a ~26% drop in airflow across the MAF, i think this is too much of a difference for even a low compression engine. If it was just compression though, the O2 sensor wouldn't be reporting way lean, if anything, it might be too rich from the raw unburnt fuel.

To me, this will always mean a vacuum leak. A tiny pinch on the gasket between the upper and lower intake manifolds for the rear lower runner dropped my MAF g/s from 5.5 to 4.6ish, and put my fuel trims to +5%. I never found it through spraying carb cleaner around, even at the correct spot.

I'd recommend having a shop smoke test your intake, find it that way.
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Old 01-03-2014, 01:56 PM
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Sorry that I haven't been getting back to you quick enough bud. Very busy this time of year with work. So i see you checked the plug tightness and confirmed that they are fine. Can you try doing one thing for me:

Monitor your fuel trims and maf voltage live if you can
tap on the maf sensor (the top of it)

its a very good indicator of a bad maf. You would see the voltage jump around.

(btw, resetting the fuel trims on a S2 is a disconnect of the battery and an immediate brake stomp [within 30 seconds])
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Old 01-03-2014, 02:02 PM
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Good advice guys.

Low g/sec and high Fuel Trims at idle are a pain in the ***, if it has always been this way - might be worth tuning it out, but if it is new - then you can bet it is bad o2 sensor / vacuum leak / o-ring around the maf might be pinched also.
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Old 01-03-2014, 03:08 PM
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I understand paimon its no problem. I will try and reset the fuel trims the way you suggested. I don't see a listing for maf voltage in Torque but maybe I am missing something.

I swapped in my old stock maf just to see if there is a difference since I had it around anyway.

I wouldn't say the ltft is new it has been this way for a long time now, same as the low maf readings. I thought it was an exhaust leak causing the high ltft at first and got some new gaskets because there actually was a leak but that fixed the leak. Someone did suggest it was a possible bad o2 sensor but I really hope that isn't the case.

I have considered buying the person mazdaedit software and having someone tune it but I don't really know the costs associated with tuning the car other than the initial investment of the cabe/software. When I get my tax return I was planning on buying some struts and springs but maybe the mazdaedit software will be a better way to go. I would also have to buy a cheap netbook to do that too.

Last edited by xexok; 01-03-2014 at 03:13 PM.
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Old 01-03-2014, 03:15 PM
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here you go
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Old 01-03-2014, 04:11 PM
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What are the AFR readings at idle? Do you have a log?
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Old 01-03-2014, 05:51 PM
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Log is in the first post near the middle. it is CSV format. I will get another tonight now that the car has had a chance to learn the fuel trim better.

Posted From RX8Club.com Android App
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Old 01-03-2014, 05:59 PM
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Can you just post the CSV into a google spreadsheet, then share that as link only? I'm probably not alone in being unwilling to download certain file types from the web without knowing exactly what is within
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Old 01-03-2014, 06:23 PM
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Google should let you look at it online I think. All CSV is is a text file of comma separated values. I will see if I can put it in a spread sheet from my phone if not I will put it up tonight at 10 or 11pm PST.

thank you for the maf voltage pid paimon I added it in.

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Old 01-03-2014, 06:28 PM
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Yeah, I know what csv is, I just make it a standard rule.

The only options for us are to add to our own drive or download
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Old 01-04-2014, 12:31 AM
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Here is after my reset https://docs.google.com/spreadsheet/...2c&usp=sharing the ltft didn't really go above 5 but it has gone up since then.

This is from my drive home tonight and includes maf voltage as well. https://docs.google.com/spreadsheet/...EE&usp=sharing

I tried tapping on the maf once the car was at a steady idle and it did nothing at all.
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Old 01-04-2014, 08:45 AM
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Just looked through them. The post-reset log is a bit more telling, as it doesn't have crazy LTFT folded in. +5% STFT with ~4.4g/s at idle. I'm still convinced that you have a vacuum leak somewhere, and I re-suggest a smoke machine test.
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Old 01-04-2014, 09:04 AM
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Originally Posted by RIWWP View Post
Just looked through them. The post-reset log is a bit more telling, as it doesn't have crazy LTFT folded in. +5% STFT with ~4.4g/s at idle. I'm still convinced that you have a vacuum leak somewhere, and I re-suggest a smoke machine test.
Yeah I'd do this, as well as double check spark plugs. At least your wideband is moving around, so odds are it isn't totally shot. Back to basics, fuel, air and spark.

Vacuum test, and make sure the plugs aren't fouled. Then if verified all is well, might be worth a tune.
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Old 01-04-2014, 10:32 AM
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From new guy...

ECU thinks it needs to add more fuel than what it has added, therefor positive trim. Are tail pipes really black, like it is running rich? If not, then ECU is doing things right. Question is, why is the extra fuel needed? Low fuel pressure? Clogged injector (not sure here, there is only one involved at idle, but maybe)? Can you toss in a can of fuel injector cleaner (I like lucas) and see if something changes?

If not, there is an ugly vacuum leak and smoke test is the only way to find it.

Have you by chance installed a catch can? If so, how?
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Old 01-04-2014, 11:53 AM
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I am working on finding a shop with a smoke machine. A shop I trust out here has one but it is broken right now. A friend is willing to take a look and try and find a vacuum leak I'm just not sure when. I may just call around today and see if I can find a shop to do it. I'm pretty convinced it is a vacuum leak as well I have just exhausted my ability to find it at this point so I guess it is time for a pro to try.

I only looked at the trailing spark plugs but they looked great still.

No catch can, and intake has always been clean. I use fuel injector cleaner every 5,000 miles or so since I got a great deal on a big box at costco.
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Old 01-04-2014, 01:24 PM
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Check your AFR when your car is cold if it's really lean then that's another good sign for your o2 sensor.
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Old 01-04-2014, 02:55 PM
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When I started my car it was around 13.6 while cold and then when warm gets up to 14.6-8.

I just did a 'backyard' smoke test and couldn't detect any leaks. I was hoping for the easy fix but I will just take it somewhere when I get a chance.
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Old 01-06-2014, 01:44 PM
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the symptoms don't always indicate a vacuum leak.

the symptoms also point to 2 other problems that coincide with each other:

low fuel volume at high loads (faulty fuel pump, fouled pickup strainer or cracked fuel pump housing bleeding pressure)
+
an intake resitriction such as a stuck SSV or non-functioning SSV solenoid/actuator(the engine would struggle near redline though and have a noticeable power loss after 6k)

Last edited by Karack; 01-06-2014 at 01:48 PM.
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Old 01-06-2014, 02:54 PM
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Karack,

I'm interested in a further explanation. To my understanding, a high load fuel flow problem will not manifest itself at idle as a consistent lean problem, and it would not reflect as a low MAF reading.

The SSV is closed at idle, but open or closed that will not cause a restriction of MAF airflow by 20%. And a restriction that is capping airflow at ~4.5g/s, or even at 80% of expected, would be causing huge airflow problems all across the rev range, and the OP isn't reporting that (and I'd bet he would have). Regardless of any restriction however, as long as the MAF is reading the correct air flow (even if heavily reduced), then it won't have it's fueling so far off. I would also expect that a Renesis that is actually only consuming 4.5g/s at idle would no longer be idling at 850rpm, but something more like 650-700rpm, but he is up at the idle speed he should be at.

Even combining those issues at the same time doesn't seem like it would produce this current result.

I guess another way of checking is to see if the ECU has to open the throttle or idle bypass at all to maintain 850rpm. If there is a vacuum leak, there should be less ECU idle control input than if there isn't. It would take a pre/post comparison on the same engine to really use this info though.



To me, the engine is still using 5.5g/s at idle. about 1g/s isn't being recorded by the MAF. Either it's a flaw in the intake stream (he mentioned removing a screen) where the MAF just isn't correctly reading the airflow that IS passing, or the MAF is reading the airflow fine, and there is air getting in through a different path post-MAF.
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Old 01-06-2014, 05:17 PM
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i'll admit i didn't read through the whole thread but mainly the original post, it wouldn't affect the low measurements at lower speeds but i also wouldn't have worried too much about the idle gram/sec measurements since all the cars i have checked aren't necessarily all reading close to the same but they all do get over 200g/s at higher RPMs/loads.

the idle AFR is still a little lean even with the trims being rather high, you're right that it is unlikely a fuel problem would probably show up all across the board.

the next thing i would probably assume would be either a voltage/ground input issue to the MAF or a faulty MAF itself.

just figured that there should be other things to verify instead of focusing on the MAF itself and possibly just running in circles since it was already replaced and apparently did not fix the problem. it's unlikely to get a new MAF which has the same problem as the original one.

Last edited by Karack; 01-06-2014 at 05:20 PM.
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Old 01-06-2014, 07:26 PM
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I will consider buying a new MAF but I just don't want to waste that money again as once the package is open there are no returns.

My stock maf that I swapped back in to test wouldn't see over about 190 g/s so I put in the denso again and that has seen as high as 207. I will look around and see if any grounds in the car need to be tightened or fixed once I get it back.

The reason I removed that screen is because it looked like this one https://www.rx8club.com/series-i-tro...screen-118307/ but before and after the idle maf readings did not change. I do plan on getting a new screen for it but my car is in the shop right now having the bumper repaired so it might be a week or so before I can do anything.

Once I get my car back are there any additional things I should be logging that could help?
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Old 01-06-2014, 08:05 PM
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I still say get a smoke test on the intake to check for a vacuum leak. Either it's going to pinpoint the problem, or it's going to prove me wrong and rule that out for sure.
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Old 01-11-2014, 11:33 AM
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replied to your pm then saw this thread

you need to normalize the compression readings back to 250 rpm, this is the result using the numbers you provided (680 kPa @ 250 rpm is the pass/fail limit):










Edit: your actual vs calculated compression values in the OP don't correlate. I assumed the original values were correct, which means the motor is shot.

.
Attached Thumbnails High positive fuel trims, low maf grams at idle-xexok-compression.jpg  

Last edited by TeamRX8; 01-11-2014 at 12:48 PM.
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Old 01-11-2014, 01:02 PM
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Just throwing this in to add to the confusion ... LOL

Yesterday I did some work on a renesis that was running 4.2ish g/s at idle (800 rpm) . LTFT was sitting at 0 and AFR at idle was around 14.7 .

The idle was smooth as silk and there were no issues reported by the owner .
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