Hesitation under load (Turbo)

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tbs1775
Posts: 25
Joined: Thu Jun 22, 2017 10:34 am
Car: S13 VH45-TT
Location: Washington State

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I found a post that sounded extremely like the problem I have (except I have no backfiring):

(engine-cutting-out-backfire-on-hard-acc ... 70-30.html)

However, their fix didn't work for me. Basically, under sudden increases in load (putting my foot in it), I get severe hesitation but no codes on the ECU, besides the codes for no cats and no speedometer. I've only got it at around 4psi boost right now and I've tried quite a bit of things to fix it:

-Subtracting timing through Nistune
-Adding fuel at various points
-Replaced spark plugs
-Checked timing
-Checked CAS connections (CAS is brand new)

Light acceleration it seems to do really well with, taking it up the rpms slower. But any sudden increase in load just doesn't work at all.

I'm thinking about trying to swap the injectors now but I don't know if that's my problem. AFR seems pretty stable under boost until the hesitation kicks in, but because of the slight delay between the actual AFR and what shows on the gauge, it's possible that any fluctuations after the hesitation starts are from me taking my foot off the pedal. I haven't tried cleaning (or replacing) the MAF or doing anything with the injectors yet. Does anyone have some info that might help me out? I'm told that the stock ECU map should be good for low boost, so between already trying the stock maps and reducing timing/increasing fuel AND trying timing/fuel maps from another person (each of those produced the same hesitation), I'm starting to think that the ECU isn't the problem.

Thanks!


ZiprHead
Posts: 110
Joined: Mon Jan 02, 2017 11:30 am
Car: 1999 Infiniti Q45 base
Location: Saginaw, MI, USA

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Really sounds to me like fuel starvation. Have you checked the fuel pressure?

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tbs1775
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Joined: Thu Jun 22, 2017 10:34 am
Car: S13 VH45-TT
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I did check it back when I was attempting to use just a FMU for fuel management, but that was about 2 years ago probably? I still have the fuel pressure gauge attached from that adventure though so maybe I'll take another look at it. What's going to be the best way to test that out? I think when I did the FMU they said to just clamp the fuel return line and see how much pressure it builds up to. It has a Walbro 255 in it right now so I'd think it would be fine but you never know, I did put that in at the very beginning of the build about 4 years ago. Granted it hasn't seen much running time but it's been in there long enough for something to go wrong possibly. The first 3 years was just me doing all the labor involved and it wasn't until last year that it started to run, and this year it's actually getting some light road time while I troubleshoot.

So what would you say is the best way to check that? I could mount my GoPro on the windshield outside and record a short drive aimed at the fuel pressure gauge, or I could just do the clamp thing again if that's a real thing.

Thanks for the reply, I appreciate it!

Edit: I just noticed also that my brake booster might have went out. If I sit at an idle and pump the brakes, my boost gauge shows pressure building which each pump, then goes back to normal vacuum pressure. Or maybe that's normal? I've never converted something from N/A so I'm not familiar with what happens with the brake booster. A quick Google search tells me I need to put a 1-way check valve in line with the booster so it won't blow out under boost. That might explain why the brakes don't work under boost.

ZiprHead
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Joined: Mon Jan 02, 2017 11:30 am
Car: 1999 Infiniti Q45 base
Location: Saginaw, MI, USA

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As I've seen, the best way to check fuel pressure is to disconnect the line from the fuel rails and insert a pressure gauge directly into the line with nothing else connected. If you're getting good pressure, you've at least isolated the problem area. If not, then you'll have to check out the pump, filter, etc.

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tbs1775
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Thanks, I'll get on that first thing tomorrow morning and see what I can find. I'm guessing that should go on the feed line and not the drain line, correct?

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tbs1775
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Joined: Thu Jun 22, 2017 10:34 am
Car: S13 VH45-TT
Location: Washington State

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I was going to edit my previous post so I wouldn't bump this thread, but since it's already at the top and I cant edit my post, I'll just post again.

I found through my searching the question I posted about where to put the gauge at in the fuel line. I also found that idle should be 34psi and about 43-44 under load. However, that was from posts that were not boosted. I've read that the stock FPR is 1:1 ratio when it comes to changing the fuel pressure in relation to manifold pressure, so does that mean I'd be looking for something like 48-50ish psi at the target 5psi of boost? Is the "44 under load" for an N/A motor assuming 0 vacuum or what should I be expecting to see at 0?

I'll be posting my findings probably by noon PST tomorrow (the 27th).

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tbs1775
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Joined: Thu Jun 22, 2017 10:34 am
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I'm a little late for my noon update but Windows *insisted* that I perform an update. Also, here's a video of one of the hesitations from the GoPro I had under the hood with a flashlight. Boost on this run didn't appear to go above 0psi during this run, I think it was actually peaking at or near 0. Throttle was maybe 30-40%, it doesn't take much more than the regular acceleration/cruise to cause the issue.


Video: https://www.youtube.com/watch?v=-M8tg6rv5t0

Note: I don't know why but YouTube is taking forever to cut the length of the video down. Eventually it will, but until then you can skip to around 2:40 and see the fuel pressure during the hesitation issue. (if the video is only about a minute long, that means the editing finally finished) Until then, I suppose you could watch all 9 minutes, there's a couple other times that the problem happens a little bit.

Quick bullet points of what I found:
- Idle pressure is around 34psi.
-Cruising pressure is high 30s to about 40psi.
-Hesitation occurs and fuel pressure fluctuates with it between low and mid 40s psi, maybe hit 50psi once but it's hard to tell.
-Fuel pressure drops to high 30's on engine shut off.
-15 minutes after shutdown, pressure still reads 38psi. 45 minutes after shutdown it is at 22psi

Other note:
-After a run I had a small leak on one of my return line fittings, I was somewhat surprised that when I unhooked the return line that only a small amount of fuel came out and also the pressure on the feed line remained the same and didn't fall. I guess that could be normal, but it's not something I was expecting to see.

ZiprHead
Posts: 110
Joined: Mon Jan 02, 2017 11:30 am
Car: 1999 Infiniti Q45 base
Location: Saginaw, MI, USA

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Looks like a fuel pressure issue to me. Sounds to me like your pressure is just marginal and when pushed it's taking its own sweet time to build up the added pressure needed.

You've already upgraded the pump. Are you still using the standard fuel filter? You might try switching to a high flow filter if so.

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tbs1775
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Joined: Thu Jun 22, 2017 10:34 am
Car: S13 VH45-TT
Location: Washington State

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ZiprHead wrote:
Thu Jun 28, 2018 3:26 am
Looks like a fuel pressure issue to me. Sounds to me like your pressure is just marginal and when pushed it's taking its own sweet time to build up the added pressure needed.

You've already upgraded the pump. Are you still using the standard fuel filter? You might try switching to a high flow filter if so.
I'm using the 300zx fuel filter right now, which should be good right? Man, what a huge PITA this is turning out to be. I guess I'm probably going to have to just go one thing at a time and replace everything in the fuel system to see if that fixes it? New pump, new filter, new injectors or have these ones cleaned, new FPR, then see where that puts me at.

I also ordered a boost controller so I can get the wastegates to stop opening up so close to 0psi, that's making it hard to see if the issue is being caused by the wastegates opening or something else because they're usually happening at the same time; the wastegates start sputtering and the engine starts hesitating.

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tbs1775
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Joined: Thu Jun 22, 2017 10:34 am
Car: S13 VH45-TT
Location: Washington State

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Just a random update, I've been reading around on other forums and it sounds like failing coil packs can cause the same kind of hesitation under load so I'm going to look into getting new coil packs and see if that fixes it. Eventually I'll just have a brand new motor.

ZiprHead
Posts: 110
Joined: Mon Jan 02, 2017 11:30 am
Car: 1999 Infiniti Q45 base
Location: Saginaw, MI, USA

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Costs money to blaze new territories. Good luck, brother.

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tbs1775
Posts: 25
Joined: Thu Jun 22, 2017 10:34 am
Car: S13 VH45-TT
Location: Washington State

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ZiprHead wrote:
Fri Jun 29, 2018 6:53 am
Costs money to blaze new territories. Good luck, brother.
That's true, thanks for the well wishes. I'll update this if and when I find a cure for the problem. Got a lot of parts on order and more to follow if it doesn't get resolved.


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