mk2davis
(Member)
02/15/19 06:25 PM
Stumble at 4k rpm under moderate load

You guys are always so helpful. I'm hoping to tap your wisdom one more time... and no, I don't have a logger. ALTHOUGH I WOULD LOVE TO BORROW ONE!

I've been getting my dad's VR4 back into DD status, and it seems to be going well with a few nagging issues. When starting from a stop:

Gentle and slow with the throttle, no issues
50-70 percent throttle, pulls fine until 4k where it loses power, keeps boost, will buck if you don't pull out
100% throttle sometimes pulls like a scalded cat, sometimes pulls like a cat in slow motion, doesn't buck but keeps boost and just isn't that fast

The long block was a remanned motor with Felpro head gasket and APR studs, BSE, 16g small turbo, max 12# boost, rewired Walbro 190hp pump, and 2.5" exhaust.
New meaning 3 months old and 700 miles: NGK spark plug wires, NGK spark plugs gapped at .028", fuel filter, fuel pump, ISC, RC Injectors cleaned and blueprinted stock injectors, OEM FPR... I think I may be forgetting some things

Base timing was set correctly at 5* before the ECU was sent for inspection by ECMTuning. FIAV blocked off. Boost leak out of throttle body shaft.

Plan of action:
Replace Throttle body seals (most TB issues I've read about are at idle. This car's idle is rock steady at 850.)
Check TPS

I've read a bad CAS could lead to this? I don't just want to throw parts at this... It's bad for the body work.
I've read about some people replacing a transistor? I don't know where or what it does.

Besides this, I'm stumped. Please help me, OBGalant Kenobe. You're my only hope...


belize1334
(well bread and nobly conceived)
02/16/19 01:09 PM
Re: Stumble at 4k rpm under moderate load

If you're confident in the timing and your plugs are gapped correctly, my next guess is fuel cut. If you're on the stock airflow meter and stock ECU you'll run into fuel cut at around 15psi (that's just a guess, I can't remember the exact value). If you've got a boost leak somewhere that will inflate your airflow reading and you'll get fuel cut even earlier. You should test the positive pressure side of your intake (turbo inlet to manifold). You can build a tester with a bike pump, a valve stem, and parts from your local hardware store; there are plenty of write-ups to guide you.

Edit after refreshing my memory: 1G MAS caps at about 380cfm, which is about 15psi on a 14b/s16g at redline. If you're getting fuel cut at 4k and 12psi it'd have to be a big boost leak, but it's definitely worth checking.


mk2davis
(Member)
02/16/19 06:18 PM
Re: Stumble at 4k rpm under moderate load

Belize, I know I am leaking boost from my throttle shaft seals. Those parts are on order. When the car is having issues, it's at 4k and any where close to any boost. And it doesn't feel like fuel cut I've noticed in the past. Fuel cut I've felt before is more sharp, where this feels like a bleed. And writing this makes me wonder if I have a leak in that z pipe that goes from the air box to the turbo? The j pipe from the blow off valve to the intake has seen better days. I have a silicone piece on order. 1 thing I should test is the accuracy of my boost gauge.

Dan D
(220, 221... whatever it takes)
02/17/19 07:25 AM
Re: Stumble at 4k rpm under moderate load

Maybe worth checking the throttle position sensor. A worn spot can cause symptoms similar to what you describe - hesitation, bucking, etc. Your foot and mind will play tricks on you, convinced it's boost or RPM related when really it's how you use the throttle in that operating envelope. Quick and easy to check with a multimeter and worth a look. That was when the car was nearly stock many many years ago - much different relationship between boost, rpm, and throttle position. It had me fooled when it happened.

transparentdsm
(I have to say something dumb Member)
02/17/19 08:27 AM
Re: Stumble at 4k rpm under moderate load

Quoting mk2davis:

Belize, I know I am leaking boost from my throttle shaft seals.




this is more then likely your issue. if its leaking boost its gonna fall on its face.


mk2davis
(Member)
02/17/19 09:32 AM
Re: Stumble at 4k rpm under moderate load

Good, then it means I'm in the right track. I didn't want to waste time if other items were suspect and should be tested.

mk2davis
(Member)
02/17/19 09:37 AM
Re: Stumble at 4k rpm under moderate load

Thanks Dan. While I wait, I'll back probe the TPS. If I recall correctly, I set it to .5v and look for a smooth increase.

belize1334
(well bread and nobly conceived)
02/17/19 03:13 PM
Re: Stumble at 4k rpm under moderate load

Quoting transparentdsm:

Quoting mk2davis:

Belize, I know I am leaking boost from my throttle shaft seals.




this is more then likely your issue. if its leaking boost its gonna fall on its face.




Agreed. Even if it's not fuel cut, you'll be running extremely rich (especially if your other hoses are leaky). The factory tune is for ~10:1 AFR. A boost leak will make it run even richer than that so you'll be down on power and possibly too rich to ignite the mixture. Before you do anything else I would make sure the intake is air-tight from the filter to the intake manifold.


mk2davis
(Member)
02/17/19 06:07 PM
Re: Stumble at 4k rpm under moderate load

So it sounds like I have a set plan of action.

Why does everyone boost leak test from the turbo inlet and not from the air filter coupler? In essence, take the gigantic coffee can off, and test from there?

I'll post my progress and hopefully a solution. It's so unfortunate that so many people, usually low volume posters such as myself, solicit help but don't always follow up with what helped and what didn't.


mk2davis
(Member)
02/17/19 07:16 PM
Re: Stumble at 4k rpm under moderate load

And just to give everyone an accurate idea, with the boost leak tester the intake system would bleed from 20-0 psi in about 20 seconds. Maybe 25.

mk2davis
(Member)
02/18/19 01:43 PM
Re: Stumble at 4k rpm under moderate load

I checked the TPS with ignition on car not running. I saw .51v at idle and 4.96 at WOT with a smooth increase. So that's one possible cause eliminated. Now just waiting on the elbow and the shaft seals.

transparentdsm
(I have to say something dumb Member)
02/18/19 03:25 PM
Re: Stumble at 4k rpm under moderate load

Quoting mk2davis:

So it sounds like I have a set plan of action.

Why does everyone boost leak test from the turbo inlet and not from the air filter coupler?




I test from the compressor housing inlet because I want to know if my blow off valve is leaking.

As for your bleed down question, I see 25 psi to 20psi takes about 15-20 seconds, 20psi to 10psi over the next 30 or so and 10psi to 0 takes forever. I usually bleed off tye coupler before ever getting to 0.

My numbers may be a little off, but I haven't needed to do a boost leak test in a while.


belize1334
(well bread and nobly conceived)
02/19/19 05:34 AM
Re: Stumble at 4k rpm under moderate load

In my experience the bleed-down time depends on how well your valves are seated and whether then engine is dead-on TDC. If you take the oil cap off you'll hear air whistling through the head. If you have a big leak in your charge pipes, it will be obvious compared to the much slower bleed-down through the head.

You should test from the turbo inlet because the intake hose from the filter to the turbo is never meant to see positive pressure; if you try to put 20psi on there you'll probably burst the hose. Testing the filter-to-turbo hose for leaks is a lot easier by hand -- just remove the pipe and detach the MAS, tape a plastic bag around the big end, cover the small hand with your hand, and use your mouth to pull a vacuum through one of the small ports. A few psi of vacuum is enough since that's the most that hose will every see in application.


mk2davis
(Member)
02/19/19 01:22 PM
Re: Stumble at 4k rpm under moderate load

^^^ This is why we ask questions, kids. Because people smarter than you will answer them. Thank you Belize, Dan, and TPDSM! I love knowing not only what to do, but why. When things come in, I'll update accordingly. If anyone has other ideas on things I should check, don't be shy. Obviously the boost leak is an issue, but I don't mind looking at other things if just to rule them out like the TPS.

Thanks everybody.


thomcasey
(I ain't no puny human)
02/20/19 05:50 AM
Re: Stumble at 4k rpm under moderate load

Do you still have a MAFT in the system?

mk2davis
(Member)
02/20/19 12:07 PM
Re: Stumble at 4k rpm under moderate load

Thom, pardon my ignorance, do you mean a MAF? If so, yes I still have it [EDIT: the stock MAF]. If not, then I don't what a MAFT is.

thomcasey
(I ain't no puny human)
02/20/19 02:10 PM
Re: Stumble at 4k rpm under moderate load

Was wondering if you were using a MAF-Translator (MAFT) or if it was the stock MAF. The Translators allow the use of a GM-MAF to VTA the BOV. They also have a fuel cut-off setting that can cause issues if not set properly. But, since you have the stock MAF, then my question is moot.

mk2davis
(Member)
02/20/19 02:26 PM
Re: Stumble at 4k rpm under moderate load

OK, that makes sense. Along those same lines, could a dirty MAF be a part of the problem?

mk2davis
(Member)
02/24/19 07:37 PM
Re: Stumble at 4k rpm under moderate load

So everyone that THOUGHT it was a boost leak was RIGHT ONE THE MONEY!

It's not perfect, but it's pretty darn good. It drives much better, comes on boost quicker, and stays on boost well. Another boost leak test went from 20-15psi in about 20 seconds, 15-10psi in another 30 seconds (50 total), and forever to 0.

Now for the not exactly perfect part. If I pull from 2nd at 1500 rpm and stay on the gas, at about 7psi or so it will lose a small amount of power, although boost stays on. If I downshift into 2nd at 3k rpm and throttle up, everything is fantastic.

Does this sound like another boost leak I haven't found yet, or possibly the WG fluttering creeping open? I have no boost controller, and the WG receives a boost signal from the intake side of the IC. I need to the test the WG to see what pressure it breaks at, but since it's rebirth I have never seen more than 12psi on the gauge.


belize1334
(well bread and nobly conceived)
02/25/19 08:27 AM
Re: Stumble at 4k rpm under moderate load

Without a datalogger it's impossible to know for sure, but if you're going full throttle at 1500rpm and it's building boost and then hesitating a bit, I'm guessing that the computer is detecting pre-ignition and then retarding the timing. Search around for 'phantom knock' and you'll get lots of info on it.

The good news is that it shouldn't be an issue because you really shouldn't be mashing the throttle below 2000rpm anyway; it's hard on engine bearings and below the boost threshold for the turbo to be of any use. Low rpms are fine for cruising and light throttle, but listen to your engine, and if it feels like it's laboring either back off or downshift. You'll quickly develop a feel for what it likes and doesn't like.



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