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Thread: Hesitation issue - need help!

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2009-09-07 02:54:23
Hesitation issue - need help!
Ok all you turbo junkies, I've got an issue I've been battling with for quite a while now, and I need some fresh advise!

The setup:
2001 Sentra SE - The motor started life as a stock roller rocker sr20 that came with the car. It's gone through a few iterations, but here's what's current with the motor:
protech log manifold
GT2871r w/T3 0.48a/r turbine housing
protech 3" dp that ends with a 16" resonator and turn down right behind the driver's seat
50lb injectors
jwt b14 ecu
fmic setup
z32 maf (blow thru setup)
emanage ultimate
eagle rods
cp 8.5:1 pistons
ARP head studs (done before reading all the bad stuff about them... :o )
stock head gasket (cometic didn't seal, didnt have time for machine work prior to a national prosolo event)
head is completely stock
innovate LM-1 afr tied into EMU for datalogging
Crane HI-6 ignition box
Crane LX-91 ignition coil (modified distributor cap for external coil)
plugs gapped to 0.028
fresh oem wires, cap and rotor

The problem:
When the car is at about 13psi or less, it runs perfectly. It pulls even and strong to redline. Above 13psi, I start to get a hesitation between 6100 and 6400 rpm. At 13.5-14psi, it basically feels like I lift a little, and then it pulls strong for the last couple hundred rpm. If I try to run 15psi, it will pull very hard up to 6100, and then it feels like I hit a rev limiter.

At first, I thought it was detonation. The car was originally on the stock b15 ecu with the EMU. I used the EMU to retard timing and control the injectors. I pulled more timing using the EMU. At one point, the car was downright sluggish. EGTs were higher, and the hesitation was still there. Hmmm, not it...?

Next I went to the plugs. I'm running ngk bkr7's. I closed the gap all the way down to 0.024. No change, but it idled rougher!

Ignition didn't seem to be the issue, unless the coil was failing, so I moved on to fuel issues. Throughout this whole process, I have been logging with the EMU. There were no visible issues with the injector duties, afr, timing, tps, or maf voltage. Could the injectors be failing at a specific duty cycle? Luckily, a local peformance shop (J Tune in Delaware) are a bunch of great guys. I was able to test a set of used injectors they had in the shop. I swapped, retuned the fuel maps, and gave it another go at 15psi. No change. Grrr.

Still not convinced it's not detonation, I stopped by a local sunoco after an autox. The tank was nearly empty, and they have 100 octane at the pump. After my most expensive tank of fuel in this car... no change. The hesitation happened in the exact same rpm range and boost. I would have expected it to at least shift with an octane increase, if it was detonation. Grrrrr.

My injectors back in, I decided it was time to step up my engine management game. I found a good deal on an obd II b14 jwt ecu and z32 maf, and now the car is running a jwt 8.5:1 compression, z32 maf, 72lbs msd tune. Since I have 50lb injectors, I used the EMU to tune the afrs (adjusted injector pulse, not maf voltage). I'm using the b15 intake still, so the iacv-aacv doesn't work. I have the idle up at 1100rpm to keep it from stalling when the fans turn on... with the loud exhaust it makes for fun looks at redlights Eventually I'll complete the mani swap and probably go to a realtime tuneable setup so I can ditch the EMU. Anyway, after all that, running on a completely new ecu program with better timing maps... the problem was still there. Grrrrrrr.

Back to the ignition. Could the coil be failing? I already had a Crane HI-6 ignition controller to help ease the redline bashing of autox (stock fuel cut was very harsh on the b15 ecu), so I added the complimenting LX-91 coil and modified the distributor cap. Note, by now I had also replaced the cap, rotor, wires, and plugs to see if they were the culprit. Again, after all that, no change. GRRRR.

So here I am. I've gone through fuel, ignition, overall ecu control, multiple variants on tuning with the EMU (used the built in injector scaling and manually adjusted fuel trims to compensate for injector sizes). What am I missing?

Could it be that the 0.48 T3 housing is too small and causing reversion issues with the stock cams and log manifold? There's no evidence of boost falling off. I'd hate to drop $300 on a larger T3 housing and still have the problem, but I can't think of anything else.

If anyone has a thought, please throw it out there. I may have forgotten a few things I tried, but I'm open to anything (preferrably things that don't take much $$ if we're not sure it is the issue). I've already thrown a lot of money at it in the above process, and I hate to keep doing it. Granted this was over quite a few months, but I have a feeling there's something simple I'm missing. Help me find it!!! Thanks in advance!
2009-09-08 01:24:23
I've been reading more about reversion and it's tendency to occure with high backpressure on log manifolds. Being that the 0.48 a/r housing, while larger than a 0.48 T28 housing, is on the small end, I'm beginning to convince myself to try a 0.63 T3 housing. I'm hoping more knowledgable turbo gurus can chime in to save me the $ or convince me to spend it
2009-09-08 12:46:58
Other than running a TINY exhaust housing on that 3071r, the only other thing that sticks out at me is the fact that you state you're using a stock headgasket because a Cometic wouldn't seal due to needing machine work.

Obviously not something you should consider keeping in the long term if you plan on running over 1bar of boost.

And get rid of that tiny .48 A/R housing! Geezus man you don't drive a 1300lb fiat .63 A/R would be just fine for your turbo. Spool time may increase a tad bit, but the powerband will be greater.
2009-09-08 13:39:24
Well, this thing's been through the ringer a few times to get ready for autox events. The last time I literally put it together the week leading up to a prosolo, drove it all over to break in, then proceeded to beat on it a Pro Solo event for 2 days. If you don't know what that is, think drag racing for 60 ft, then turning into an autox course. Had I pulled everything, sent it to a shop to deck the block and head... I'd have missed the event. At some point I need to get back in there and freshen it up, especially if I get a bigger A/R and push it.

The small A/R was the result of two factors. First, when I bought the turbo 3-4 years ago, I didn't know what (I think) I do now. Second, I talked to the shop I bought it from and told them I want quick spool to save lag time during autox. I was also on a stock motor back then, so wasn't looking for more than 250whp.

You think the A/R is causing the issue? It's not losing manifold pressure at high rpm, even when it hesitates, so the compressor can keep up.
2009-09-08 15:17:31
I hear you on putting it back together at the last minute in order to make the event. Happens to us all Just as long as you know it needs to be looked at, that's good enough.

I'm not very familiar with your specific setup (the Crane external coil setup), but it sounds like that is what the issue is. I know Jamie was running an external coil setup (don't know if it was Crane or MSD), and he had nothing but issues in the higher RPM's in boost. Felt like ignition cut or fuel cut. He got fed up with the setup and ripped the external coil setup out, went back to an OEM OBD-1 external coil distributor on his B14, and his problems disappeared almost immediatly.

I would think that this is probably what you are experiencing? Seems I'm seeing more and more of these almost identical issues with external coil setups, lately. May be worth a shot to switch back to an OEM external coil dizzy and see if it solves anything. Have you checked the cap and rotor since you replaced it?
2009-09-08 16:23:40
Hmm... Well it's been doing this since before I put the external coil on. I actually put the coil on thinking my stock coil was going bad, and it was less expensive to "upgrade" to an external unit than spend the cash on a new b15 distributor base. The car actually feels smoother with the new unit, too. At least everywhere in but when it hesitates. :o

I have had the crane hi-6 box on for a while (running the stock coil). Maybe it's something going on there... but at anything less than 13.5lbs there's no rpm dependent issues. I wouldn't think the controller would have a boost dependency. Easy enough to check, though. Just a few wires to rearrange.
2009-09-08 17:08:03
Well under certain boost ranges, the ignition shouldn't have a hard time providing spark. The more boost you throw at it, the harder the ignition system will need to perform in order to keep delivering spark. If it just isn't up to par, the spark gets blown out before it can ignite the mixture.

Again, just throwing ideas out there, as I don't have a vast knowledge of external coil systems. :o
2009-09-08 18:42:44
I hear you. Usually the spark energy is dependent on the coil, but if the crane box is sending a rediculously short ignition signal, the coil may never be able to build the required voltage differential to ignite the mixture. I'm gonna try taking it out of the system. It's free and shouldn't take long to do. Hopefully I'll find some time tonight.
2009-09-08 22:37:06
I've had reversion issues. HAHAHA! Ya the .63 is the smallest you would wanna run and the log manifold will compliment it nicely. The .82 is the best for overall power... by a lot. With reversion, you just make a lot less power throughout the powerband. Get boost pressure as close to backpressure as possible, and reversion will go away. The log manifold will be your restriction near 400whp, but even at that power level I would expect the .82 to make 30whp more.
2009-09-09 02:25:28
Maybe its because you arent even tuned for your setup. You are running a GT2871r with 50lb injectors on a 75lb tune. Do you know your air fuels?
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