RPM studder at WOT
 

I have searched a little bit and I didn't find an issue similar to mine.
I have massive RPM studder when I go full throttle (Wide Open Throttle) from 1krpm to 2.5krpm. The rpms seem to bog down unless I let off on the throttle a slight bit.

The BR OBDII scan tool shows my TPS at WOT is 88% and closed throttle is 9%.

There is a slight idle hunt after the truck is warmed up and in closed loop mode. It may be vacuum leak somewhere (TB gasket or manifold gasket).

Just poking around for any ideas or similar experiences.

Try hunting down the source of that vacuum leak by running your fingers around the edges of vacuum tubes while the engine is running. When was the last time you cleaned your MAF sensor? Also, have you done any fuel mods? (e.g. walbro fuel pump or fuel management mods like a URD or TRD 7th injector kit)

Personally I think it's lean out on your engine bc that supercharger is asking for more fuel than a stock system can keep up with.

Ding! (that'd be my guess too)

Yup, do the fuel upgrades. Your TPS is within spec, I went through the whole "It doesn't read 0 to 100%". The TPS isn't the problem. Good luck!

Ditto, the TPS is reading correctly.
Sounds like the high RPM leanout to me causing it to misfire.

No ECU codes thrown up.

The set-up I bought had gen 1 SC, rising FPR, and everything else appears to be stock. The dealer didn't know what was installed.
Now, walbro 190lph fuel pump is installed from URD kit. The injectors will be installed after I tune the FTC base map for my truck.

I had no issues with the set-up. I replace the timing belt at 90k and supercharger rebuilt around 100k. After my timing idler pulley 2 failed and I replaced the timing belt (and pulley), I noticed this studder/hesitation. It totally feels like fuel cut in a car, but much faster duty cycles.

At first, I thought it may have been transmission related since the rpms would very slowly start to rise even though the truck wasn't going anywhere. Upon closer inspection, the rpm looks as if it's studdering.

The OBDII scan tool didn't show any short term fuel trim that indicates lean condition (it was "-" and trying to pull out fuel).
I see the idle hunt as part of this problem since idle hunt is also post-timing belt replacement.

If it's fuel or timing related, shouldn't I see it when I was datalogging with the BR OBDII scan tool (ignition and short term fuel trim)?

btw, is there a convenient way to import the datalog into excel spreadsheet?

Thanks for the suggestions!
J

oh yeah, I noticed my driver side airbag warning light up for the first time yesterday. It only lit up during one drive (work to home).

A couple of things...

• The ECU won't necessarily throw a code
• Fuel trims don't come into play since the ECU is in open loop
• "Fuel cut on fast duty cycles" would be lean out or you're bouncing off the rev limiter
• What RPMs are you seening the symptom at?
• Installing the injectors AFTER a base tune will throw off the base tune
• Having the Walbro in there with the stock injectors isn't really helping anything. You're just putting more pressure behind an injector that isn't large enough to provide what you need.
• What piggyback are you using? What does your fuel map look like now?

Fuel map is 10's right now. The original map was too lean.

It's odd that this behavior happens after I replaced the timing belt. That debug effort led me to replace sparkplugs, pull fuel lines, inspect fuel injectors. Just wondering if I broke something in the process.

URD ships the FTC with a fuel map that thinks you'll be running a Walbro 190 and 305/315 injectors. So yeah, it'll be lean on stock injectors.

You REALLY need to get the fat ones in there. I can guarantee you that you're leaning out up top. I ran a Walbro on stock injectors for about 3 weeks. My EGTs and LM-1 weren't lying when they told me what a nightmare I was creating.

On the FTC, "10" is baseline, so right now you're not adding any fuel to the system. That's bad. Again, the stock injectors won't completely satisfy the S/C requirements, but you can at least make things better by cranking up the fuel map in the sections where the ECU is in open loop. If you don't, you're just adding to the lean out issue.

What's the timing map like?


Maybe... but given how easy it is to adjust the FTC, I would suggest that we play with that a bit first.

The same behavior happened on my base map of A: 10's and B: 0's.
I tweaked a bit under 3krpm, but I noticed my truck stalling on deceleration off throttle with accessories running on vacuum.

So now the truck is running with Gadget's B and A: 10's.

I understand that the stock injectors are not sufficient for the SC set-up. However, I had no issues running WOT before the last timing belt replacement. It may have been running lean, but it didn't bog or hesitate. My idle was solid and never hunting.

I want to make sure I'm not masking any issues as I add mods. My experience is the more mods you add, the more areas of failure to investigate when things go wrong. So I'm trying to stage mods on the truck.

I found the oil vent breather tube didn't have a strong connection.
Also, when I swapped out the injectors, the seals were a little worn out (from previous time when I inspected the injectors). I had a leak somewhere (fuel or vacuum) and it's fixed now.

Anyhow, put in the new URD injectors and denso sparkplugs (2-steps colder) last night. I'll post some pics of my make-shift wiring cover (low grade silicon tubing) for the fuel injectors.

With Gadget's map loaded, the truck runs extremely well for a base map. The short term and long term fuel trims are better (since the map is made for the injectors).

I also have WOT now, but with some margin due to the new injectors.

I still have signs of low idle (as low as 580rpm). Guess I need to keep checking around for a vacuum leak.

J

Thanks for the suggestions. I've been out of town for a little bit (Montreal F1 race was awesome!).

-I may be a little lean WOT during this hesitation period, even with the big injectors installed.
-The stumble/studder/hesitation or what not happens at idle, warm-up, and partially during acceleration, but it's only obvious when at idle after the truck is warm and the ambient air temperature is about 88F.
-Hesitation period starts at 2krpm and ends around 3krpm. So the truck really wants to move after 3krpm.
-Partial throttle (up to 80%) at low boost (1-2psi) or 0psi results in faster acceleration between 2krpm and 3krpm.

Could all of these symptoms come from inaccurate timing (timing belt off by a tooth)? I'm pretty sure I fixed the vacuum leaks when I zip-tied all my vacuum lines.

J

Welcome back!

Did you reset the ECU after putting the injectors in?
How many miles have been put on the truck with the new injectors?

580rpm is okay for idle, but it should be smooth.

I think you'd have more critical problems if you were off a tooth on timing. What does the BR3 show for timing advance? Anything horribly abnormal?

How old is the O2 sensor?

It's humid in Texas right now as well, right?

I had a thought on the way home... Your TPS is reading in spec, but those are only endpoint readings... There's nothing to look for "jitter" on the way from one extreme to the other.

I remember a bit ago when I had my TB off. I figured I'd clean it out real good, and used intake cleaner... a LOT of it. Afterwards, te truck ran, but had all sorts of weird flat spots and if you were cruising town at around, say, 2500 RPM in 3rd, then the engine RPM would flutter.

Hindsight being 20/20, I'm pretty sure that I'd gotten goo in the TPS. I tore things apart again, shot a bunch of _electrical cleaner_ into the TPS, and things cleared up.

Bottom line - I wonder if your TPS is dirty?

Yeah, good point. It may be the TPS. I'll check it again this weekend.
I also know my O2 and A/F sensors have not been replaced under my keep and I doubt the previous owners changed it since I bought the truck with a little less than 60k miles. I had a theory that those sensors may be dirty or close to failing, but not quite dead yet (otherwise CEL would be on).

Nothing really strange on timing advance on BR3. I did notice that this hiccup or studder at idle would result in a slight advance in timing, but it wasn't consistent.
Sometimes timing is fine, but I can hear the studder.

I also meant to clear up the terms used to describe my problem.
There is idle hunting during warm up (first start) and when it's really hot and I'm at idle with lots of accessories on (radio, AC, after hard braking). I've also noticed that the behavior slightly changed from the initial injector install. I did reset the ECU as well.
The behavior slightly changed as the idle is more consistent when the AC is running. It used to be different, especially on an incline (my truck stalled twice at a stop light when I was on a hill).

Another thing I noticed that is consistent is the idle hunt off throttle.
So, if I'm at 3krpm or so and I release the throttle and brake, I can see the tach bounce around a bit (hunting) until I stop (idle) or accelerate again.

The flat spot is definitely WOT from 2krpm to 3krpm.

Thanks for the responses.

My truck finally has an error code, P0171.
Strange that the system is detecting lean condition when my fuel consumption has signifcantly increased after the computer and fuel injectors from the URD kit were installed.

I can increase fuel in the FTC, but I would imagine the map provided would easily cover the low boost range. I'll bump up the values, but I have concern on fuel delivery (i.e. FPR).

Update:
I bumped up the fuel at 1psi to 3psi to 10 (with large injectors installed) and 3psi and beyond to 10.5 (big jump for testing).
After I wrote the ECU map, it seemed to perform better under boost and WOT.

However, the next ride (a few hours later), it was starting to have the same symptoms flattening out in acceleration when WOT except my boost gauge showed spikes from 0 psi to 10 psi. No CEL, but one of my initial thoughts is that if there is pre-ignition detonation, it could cause the spike on the boost gauge since the fuel could be igniting before the closing of the intake valve or compression.

I'll lower the fuel back to the previous map to see if I observe any changes.
The TPS and MAF will get cleaned tomorrow when I get more electronics cleaning solution.

Even with the fuel map A increase of about 3% from 0psi and higher on all rpm ranges with the Walbro 190lph fp and 318cc injectors, the truck feels like it's going lean when attempting to go WOT. The acceleration is flat and I can hear knock as if the engine is about to stall.
Looking at BR log for the run, the fuel trims show that the ECU is trying to add more fuel (+19.2:+19.2 on short:long term).

Exactly. This happens on mine when the timing's advanced too far. Which leads me to think that you may be pushing too much advance...


WOW. Umm, you're pulling the EFI fuse after you make these changes, right? Where in the range was it adding all that fuel?