Low Rpm Lean Pop Trouble

Discussion in 'Fox 5.0 Mustang Tech' started by SloNoMoRacing, Nov 19, 2013.

  1. My fairly new build has got me stumped with a lean pop at partial throttle driving, or in neutral at low rpm. At WOT it runs like a raped ape. No detonation whatsoever, but the plugs are snow white every time I pull them. I've even cut the threads off to completely to expose the lower porcelain and it's all snow white (no particulate to indicate detonation either). I know this is a fairly common problem, but the usual suspects aren't the issue here. I've been up and down with this thing and I think I just need some fresh input.

    It's basically a stock short block 302 in a 1989 Mustang GT. The relevant mods are GT40X heads (58cc with mild port blending), GT40 upper and lower intake, Compucam 2031, 1.72 Crane roller rockers, 65mm throttle body, Kirban afpr at 42 psi (without vauum), 24lb injectors, stock fuel rail, Walbro 255 lph in tank pump, C&L 75mm MAF with blue sample tube, C&L intake elbow, stock airbox, complete EGR system delete, MAC equal length headers into a 2.5" Prochamber and Pro dumps, SCT 4 bank Eliminator chip with a mail order tune for all of the aforementioned parts.

    I've played with the fuel pressure a bit, and the less I give it the worse the problem gets. 42 psi is where the popping starts to go away, but adding more fuel pressure doesn't help it any. I've taken it up to 46 psi without vacuum.

    I've pulled the codes, and none exist. Code 11 every time, in every tune position except the factory tune. This problem also existed before I installed the tunes, and whatever Bama did helped this problem and a few others, including low rpm bucking and not wanting to idle. I have done a cylinder balance test with all of the tunes, and I get Code 90 every time... I've even done it down to the third, most stringent parameter.

    I reinstalled the factory airbox, in lieu of a Moroso CAI because this problem was MUCH worse with the CAI installed. As soon as I put the factory airbox back on the engine was just happier. I was leaning towards a MAF issue, but I've tried multiple known working sensors and none have shown any improvement. I have also tried clocking the C&L unit, and also tried shifting the factory airbox to try to get more consistent airflow over the sample tube.

    o2 sensors are switching, and are grounded perfectly.

    Distributor has been swapped with a known working unit, and so has the TFI module. Crane Cams Fireball ignition coil is working too.

    I've been using AGSF-32c Motorcraft plugs, gapped at 0.044 since day one. I recently switched to Autolite AR103's gapped at 0.052 thinking that the step colder would help. I think it may be worse though.

    Anyway I'm stumped. Any help is appreciated. The only thing I can think of at this point is that the C&L meter is total crap. I've alway used Pro-M meters, but this is all I had for this build. Should I try a black sample tube?

    Thanks in advance!

    edit: I forgot to mention the timing... Timing is at 12 degrees initial right now, but the weird thing is that no matter how much timing I give it, it won't ping or detonate. I had it cranked up to 20 degrees initial one day just to see if I could get it to detonate and it didn't at all. Maybe it's just too quiet to hear over the exhaust now, but I remember when this car was bone stock I couldn't get anymore than 12 degrees without hearing the marbles in a tin can sound. It's got a 50oz SFI approved damper from Ford Racing btw.
    #1 SloNoMoRacing, Nov 19, 2013
    Last edited: Nov 19, 2013
  2. my 103' s are gapped around .035 ish, my Gt40ps are milled 30/1000 though, you tried going down??
  3. Thanks for the quick reply. Actually I haven't tried a smaller gap yet. I figured with the colder plugs I would need to increase the gap a bit, but after driving around today I noticed 2 things: 1. The pop/bucking is much worse at 1800 rpm than with the hotter plugs, and it backfires like crazy (much worse than before anyway) while decelerating; and 2. The exhaust has got to be half as loud as it was with the AGSF-32c's.

    I'll try closing the gap to .035 and report back tomorrow.

    I also forgot to add in my first post that I did a compression test, and every cylinder was between 148-152 (all within 10% of each other).
  4. cool, hopefully a smaller gap will be the answer :nice:
  5. Observations:
    1 Mail order tune (never imho)
    2 C&L Meter (Junk)

    I would start with a new meter and then have the car tuned by a local reputable shop and not someone who can not actually put their hands on your car. While I have had remote tuning sessions with Mike Moran in the past, He is still seeing all of my data in live time and my tuner is still present and on the phone with him while this all happens. (Far from a mail order tune imo and I would NEVER mail order a chip for my car)
  6. I'd start by removing the mail order tune.

    You have the EGR deleted, and i assume other smog/emmissions equipment gone as well. Is the smog pump gone?

    In that case, you should have some codes. Even the most perfect running cars on this site rarely display three 11 codes when running the codes.

    A custom dyno chip would be a much better investment than a mail-order tune.
  7. Unfortunately that didn't change the lean pop at all. Somehow the exhaust smells much more stoich though (eg it doesn't burn your eyes or nose with that lean smell).

    1. The lean pop at low RPM was present before the tune. I was skeptical at best about getting a mail order tune, but it has surprisingly done nothing but help on all ends of the spectrum (except for fuel mileage haha). The pop was much worse before the tune, and the low rpm bucking and surging has diminished dramatically, if not completely. The only reason I got the tunes to begin with was because they came with the SCT chip from AM. I will absolutely be scheduling dyno time as soon as I tweak a couple of my bolt-ons. I'm sending my lower intake to TMoss for some port work, and it looks like I may be replacing this C&L meter with a PMAS.

    2. That's what I'm thinking too. ProM is back in business, so I may have to give them a shout.

    All emissions have been deleted, and properly plugged at the lower intake. I'm pretty sure the smog pump was the first thing I removed from this engine when I got the car from my uncle in 1997.

    There are NO trouble codes, except for in the stock tune bank. That being said, the car hardly runs on the stock tune. Last time I checked the codes in the stock tune I pulled codes for all of the deleted EGR components, code 41, and code 91. Obviously those are lean codes. The EGR has been tuned out on all programs. All of the actual programmed tunes yield a code 11 for KOEO/KEOR, and code 90 for the balance tests. So... there are no codes to report.

    I will absolutely get a custom tune when I finish working out some bugs... like the lean low rpm pop that has been present since the first time started the engine after finishing the build.

    My next step will be to probe the MAF signal return wire while the engine is popping, to see if it's acting erratically. Any other suggestions?

    Thank you all for the fast input btw!
  8. Does it do it just sitting at idle?
  9. I don't think so, but it would be really hard to hear it over the lope of the cam if it did. The final lift (at the valve) on the cam is 513 intake, and 529 exhaust to give you an idea. Idle is set at 750 rpm and it maintains it pretty steadily. When you give it partial throttle, as soon as the lope starts to smooth out around 1100 rpm, that's when you start noticing the popping. It just gets louder as the rpm increases up to around 3,000. I don't know beyond that since I haven't free revved it any higher. There's no rhythm to it at all, it just sounds like an intermittent miss.

    What concerns me is how truly white the porcelain is on the plugs when you pull them out after a full throttle run. Again though, full throttle is smooth as can be. That being said, since I decreased the gap today the porcelain on the plugs is more of the nice golden tan color I'm used to. It didn't help the pop at all though.
  10. You may want to try backing down the timing to 10* base. Most tuners assume or recommend the timing is at base when they make the tune. Its also free for you to try.
    f8tlfiveo and Grabbin' Asphalt like this.
  11. Yes my tune has it at 10* also, great point and it's free. I tried to recommend something free earlier but it didn't work. Always better to do all FREE stuff first :nice:
  12. Agreed on the free troubleshooting steps. Always better than throwing parts at it.

    I haven't tried backing the timing down below 12 degrees, so I'll try it out tomorrow and report back. I'll also check to see when full advance comes in while I'm at it to try ruling out any distributor issues. I had a buddy who's stock distributor advance was screwed up, causing the timing to jump to 35 degress at 1500 rpm and continue advancing all the way up to the redline. It was pulling 70 degrees at 4000 rpm. Detonation galore.

    Thanks again!
  13. C&l with a tune..nah.... Get a slot meter and call it a day.
  14. Reduced the base timing to 10 degrees from 12, and it actually got worse. Now it stumbles flat on its face. I tried going the other way, and everything seemed to smooth out around 18 degrees initial. The popping smoothed out the higher up I went, which makes no sense to me.

    I noticed that as soon as I plugged the spout connector back in, the base timing jumped to 20 degrees while idling. This was in all tunes including the stock bank, with initial timing set at 10 degrees.

    The saga continues
  15. Definitely odd, it shouldnt fall on its face at 10* timing if the tuner used the timing tables like he should have. Its really hard to say without looking at what was done.
  16. It falls on its face even in the stock bank though, meaning the computer all by itself with no tune. Granted, it runs like crap on the stock tune since the EGR values are missing, but that doesn't explain the pop/miss at idle and part throttle. I'm 100% sure that this issue is not due to the SCT tune. Like I said earlier, this issue has existed since before I even installed the chip.

    I am more and more convinced that all of you are dead on with the C&L being crap though, and it might even be the only cause of this issue. While I was messing with the timing, I pushed down on my stock airbox and the idle speed changed. I pulled back up on it just for a second and the idle speed changed again and the engine almost died. If the C&L meter is that finicky with variations in air turbulence, how the hell can it accurately measure incoming air when the engine moves under load and the mass air meter doesn't (meter is bolted to the stock bracket on the fender apron).

    I wish I had a ProM calibrated for 24# injectors sitting around that I could swap out for a minute. I have one for 42# injectors, but that's obviously not gonna help me here. Added to the Christmas list I guess.
  17. I'm having identical issues with my set up. Light pedal under 2,000RPM drives you batty. Forget about lugging it down around 1,000RPM, because it's down right undriveable. Get popping and backfiring in the intake when it's cold, which mostly goes away after warm up. Doesn't idle very nicely till warm either. I too have tried everything you posted above and more. At this point, I'm blaming it on out of spec O2 sensors.

    I've got a calibrated (30lb injectors) Pro-M 80mm MAF unit that I thought might be causing me trouble, so I sent it out to be inspected and recalibrated. Guess what....no difference. All codes checked out. Played with fuel pressure in both directions....no joy.

    I do have new O2 sensor, but they're aftermarket (NTK). I'm going to pull them and replace them with a set of OE Motorcraft sensors and see what happens. A marginally bad, or out of spec O2 sensor won't show up when running codes....but it will make you want to pull your hair out!!!
  18. Can you stick a timing light on it while its popping? And you are using the flat spot on the timing pointer and not the circle correct?

    If could very well be the C&L. The whole process in how the Cal'd meters work, effects Load. The ECM sees it as a lower load than the engine is actually at, and most of the lower loads have a higher timing curve. So it could have too much advance until it gets into the higher loads where Ford was pretty conservative.

    Id also like to see what your vacuum reading are if you have a gauge to do that.
  19. I'd give lee @ C&L a call and have a talk with him. I'm not a fan of their MAF meters and I'd be guessing that is the cuplrit. Is there a Pro M or other MAF you can try out and see if the problem resolves?
  20. Gearbanger101: Definitely let me know if the new Motorcraft (still made by Bosch?) o2 sensors help you out. I have a pair of Motorcraft o2's on mine now, but they aren't new. They were known working units from my dads 93 Cobra with about 10,000 miles on them. They did sit on the shelf for about 6 years before I put them in though, so I can't totally rule them out. Post back here if you figure out your issue!

    liljoe07: The timing is pretty steady at 20 degrees while it's popping under 2,000 rpm. It clears up dramatically over 2,000 rpm, which is where you can see the timing comes in more aggressively. I'm not going to take what you said personally about where to check the timing, because this is the internet and you don't know me personally, and have every right to assume I'm an idiot... That being said, I did get a kick out of your question haha. Yes I'm sure I checked the timing properly.

    What you said about the C&L makes complete sense, and is just more ammo against the meter causing this issue.

    Thank you for reminding me about the vacuum, I forgot to mention that earlier. There are no known vacuum leaks on the engine. I've smoke tested, and sprayed ether on just about everything to double check it. For a cammed motor, I have noticed that this thing pulls an insanely high amount of vacuum... about 25 in/Hg at idle. It's so much vacuum, that even though my fuel pressure is set at 42 psi with the vacuum off, it drops to 29 psi with the vacuum applied to the regulator. I've never seen so much vacuum on a cammed 5.0. I've always seen more like 15 in/Hg at the most. I did try running the car without the vacuum hooked up at the FPR (plugged the vacuum line), but it made no difference either way. Thoughts?

    mikestang63: The last time I spoke with anyone at C&L, I hung up feeling like they didn't know squat about tuning. All he wanted to do was tell me how they've never had any complaints about their housings, and that it had to be my electronics that were to blame. I've tried multiple MAF electronics on this housing and they've all done the same thing. Since I last spoke with them over the summer, I can't get through to anyone on their (256) 882-6813 number. Do you have a way to contact Lee directly?

    Thanks again everyone.