Need help identifying this weird thud/

[KMOSES]

The other day i was cruising the highway, and decide I'm gonna get on it a bit.. just for a the fun of it.

here's what's happening,

If i step on the gas pretty hard, not fully WOT... just slamming the pedal from point A to point B almost instantly, boost psi raises fast jumping almost instantly up to about 14/15 psi, i get a weird thud (sounds/feels like its coming from the engine) for a split second then boost qucikly drops to 10/11psi holds there and everything is fine, the thud feels similar to hitting a bump in the road or something of the sort, but I know its not a bump in the road becuase i can relpicate the process over and over, and i feel it, its definitely happening under the hood.

this does not happen if I gradually press the pedal from point A to point B, even if i do it fairly quickly.

is this something I should be worried about, or am I just a retard and I should never jam the gas pedal as mentioned in the first scenario.

 

[DSMTyson]

Motor mounts? Idk man, that is weird. Maybe someone with more experiance can help you at.

 

[Absit]

Check for boost leaks, sounds like you may have just created one.

 

[KMOSES]

So I think i may have narrowed down to whats happening, I took it out on the highway again on my way home from work yesterday..

this thud only happens when the boost reaches around 14/15 psi, and if i try to keep on the gas and keep the boost coming it keeps doing the thud ( thud, thud, thud, thud, ) until i let off the gas and let the boost drop. the car also jerks slightly on each thud.

also, after about 15-20 min of driving home it stopped doing the thud and i could hold boost at 15psi for a few seconds with no problems and it would just slowly drop back down.

could this be due to high boost when the system and hoses haven't had time to warm up fully? temp gauge was at the norm halfway in all tests

It's sounding more and more like a boost leak or boost problem. I'll be doing a BLT this weekend, until then ill try to lay off the boost.

thanks everyone for the help!

 

[KMOSES]

Did a boost leak test, found a leak that i thought i heard coming from the throttle body but fixed one of the lines that was ripped/torn nearby and the sound coming from the throttle body went away, maybe it was some sort of echo, who knows, good news is that I can't hear anymore leaks.

The problem still persists at high boost 15-16psi i get a repeating thud/jerk/studder

I was doing some searching on the forums and was reading that stock 2g should run 10-12psi maybe 14psi, but nothing higher, anything higher will cause problems.

Mods are: autometer Boost gauge, Injen Intake pipe, HKS BOV, HKS cat-back exhuast 2.5"(i think) haven't measured it, and all the rest is stock to my knowledge.

What could be causing the extra-high amounts of boost?
Is this High boost causing my studder/thud?
How can i turn it down?

 

[95TalonOwner]

Your knocking/studdering is probably pre detonation from running too much boost with no supporting mods. If that's the case, it's not good for your engine.

 

[KMOSES]

Your knocking/studdering is probably pre detonation from running too much boost with no supporting mods. If that's the case, it's not good for your engine.

Thanks for the reply.

So sounds like the previous owner prematurely upped the boost.

Id prefer to drop boost back to stock until i have the right supporting mods.
is a manual boost controller in order? If not necessary, whats the best method to setting the boost back?

 

[RT-AWD]

You don't currently have a boost controller? Maybe your waste gate is sticking?

 

[KMOSES]

You don't currently have a boost controller? Maybe your waste gate is sticking?

I do not have a boost controller, at least not to my knowledge. where are they usually located? I'll double check to look for one. I can post a pic of whats under the hood if you want to take a look.

I'll do some searching on waste gates, and see what I can figure out about that. Thanks for the lead.

 

[RT-AWD]

a boost controller will be attached to your waste gate and intake manifold via lines. usually on DSMs they are found near the front of the car near the intake pipe.

 

[LandoAWD]

Keep in mind that an MBC will not lower boost past WG pressure.

 

[KMOSES]

a boost controller will be attached to your waste gate and intake manifold via lines. usually on DSMs they are found near the front of the car near the intake pipe.

Ok, Im 99.99% positive that i do not have a boost controller, that leaves' me with the waste gate sticking shut causing a buildup of boost pressure, thus causing the studder under high accel/boost?

or a boost leak I didn't find?

Here is a pic of the engine bay:

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[1990AWD]

First thing I see is your BOV is your BOV is venting to atmosphere. Recirculate that thing!!

If that BOV can't be recirculated (I have no idea if it can or not) get yourself a used 1g BOV and crush it.

Since I can't see in the picture, what does the vacuum line from the wastegate lead to?

 

[KMOSES]

Recirculate that thing!!

Haha Yeah, I have a recirc kit on order, will be recirculating asap.

I'm still pretty new to this. please excuse the newbieness, Im still learning

where exactly is the waste gate? what does it look like?
(I'll do some research and try get familiar, but any help is appreciated.)

what should the line from the waste gate be leading to? Will pictures help?

thanks.

 

[ktgurl87]

The wastegate is the sucker sitcking off the turbo... right next to the intake of the turbo. On your car (since it appears to still be fairly stock)that line should be leading to the Boost Control Solenoid (BCS) underneath your intake. It's a cute little black plastic sucker that has 2 lines coming off of it. On the GST we have, one line goes to the wastegate, one line goes to the intake pipe. With that intake pipe you have, I don't think there is a spot for that line, but I could be wrong.

 

[rabenne]

your wastegate is built into the exhaust side of your turbo assembly. you will see a small diameter shaft that looks like a valve plunger (because it is) this is attached to a vacuum actuator, these are the parts of your wastegate that you are interested in right now. your wastegate opens to allow exhaust gasses leaving the engine to bypass the turbine to keep the shaft at desired/safe levels. sounds like maybe just maybe your wastegate isnt hooked up to vac. when the exhaust pressure builds high enough in your exhaust manifold it will eventually overcome the spring in the wastegate assembly and open up (lowering boost instantly) and probably causing some strange tune issues and engine shaking. You should definately check your motormounts with a prybar (try to move the engine around). you want very little, to no movement. also try to see if the rubber inserts in the engine mounts are still in one piece.


ktgurl87, u beat me to it = )

 

[KMOSES]

Awesome thanks I'll take a look on my lunch break here in a few hours and see what I can find out for you all

also, i don't know if this will help diagnose the problem, but this morning on the way to work i didn't let the car warm up all the way, while quickly boosting at a little under 10psi I got the studder effect. I live in Ohio, cold temp's outside right now, just about 35 degree's.

Just to re-iterate, this studder only happens under hard accel and when the boost pressure builds very fast, more frequently when cold. It seems the car can handle much more hard accel and boost pressure once everything warms up. but still studders around 15/16psi once warm.

thanks again everyone

OK so i checked things out, took some pics, best shots i could get without taking everything apart.

i made a sort of diagram to help follow the lines.

Waste Gate Actuator Line tees off, the red line goes into the BCS i believe, then blue goes from BCS to the intake pipe. Im not sure where the green half of the WGA tee leads to. If the green line is essential to knowing where it leads, I can figure that out after work today.

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thanks.

 

[KMOSES]

I have been tracking my MPG and been getting really low numbers like 19-20 mpg on 95% highway driving, maybe the reason I'm getting bad mpg is contributing to the boost studder?

 

[KMOSES]

Hey I have been doing more searching and stumbled upon a thread over on the other dsm forum "DSMTalk". they were talking about a thing called Spark Plug Blowout, and it has a distinct sputter, and caused from the plugs not being gapped properly and being blown out by the high air flow under heavy load.

does this sound legit to you guys?

I'm gonna get some BRP6ES tomorrow and replace them and do my best to gap them properly at .028 and ill report back here and let you know if it fixes.

 

[XiKeiyaZI]

Thud....Rapid fire? Sounds like you're starting to lack fuel. I can't believe that I'm about to sound like I'm preaching this, but look into how to Rewire your fuel pump. My friends GST was 'stuttering' pretty bad at 13PSI and 4500+RPM's. We rewired his pump and no more stutter. It was a very audible and noticeable stutter.

 

[Bluefire16]

The wastegate is the sucker sitcking off the turbo... right next to the intake of the turbo. On your car (since it appears to still be fairly stock)that line should be leading to the Boost Control Solenoid (BCS) underneath your intake. It's a cute little black plastic sucker that has 2 lines coming off of it. On the GST we have, one line goes to the wastegate, one line goes to the intake pipe. With that intake pipe you have, I don't think there is a spot for that line, but I could be wrong.

The BCS is by no means a "cute little black plastic sucker". The BCS is the weapon created by Satan himself to destroy the fun and joys of boost on a 4g63.

Anyways, yes the spark plugs could be a culprit. Make sure they are gaped correct. When i started running high boost, i had to make the gap smaller.

But check them, every now and then they should be adjusted and checked because the gap can go out of whack on their own

 

[Calan]

...If i step on the gas pretty hard, not fully WOT... just slamming the pedal from point A to point B almost instantly...

Ehhh... how do you slam the pedal without going WOT?

These problems are really hard to diagnose over the net because sound descriptions are so subjective. From what I've read here, it could be anything from a carrier bearing to fuel cut, depending on your personal definition of a thud.

Spark blow-out sounds (feels) like a machine gun; it occurs as very rapid pulses. True fuel cut literally feels like you hit a wall, and there's very little doubt about it when it happens. It could definitely be ignition related, but I kind of doubt it at your boost level assuming the plugs and wires are in good shape.

This really just sounds like a tuning issue or something going on with the fueling during initial throttle input. I didn't see anything mentioned about it, but what are you using to datalog and tune with? If the answer is "nothing" or "I don't know", make that your number one priority. It's much easier to resolve these issues when we can actually see what is going on with a datalog, rather than reading subjective descriptions of sounds.

 

[XiKeiyaZI]

Spark blow-out sounds (feels) like a machine gun; it occurs as very rapid pulses. True fuel cut literally feels like you hit a wall, and there's very little doubt about it when it happens. It could definitely be ignition related, but I kind of doubt it at your boost level.

I would just have to add, that if the plugs are improperly gaped then you could simply have a spark issue. I wouldn't toss spark blow-out through the window until you confirm the condition of plugs, plug gap, and wires.

 

[Calan]

It could definitely be ignition related, but I kind of doubt it at your boost level assuming the plugs and wires are in good shape.

Fixed.

 

[XiKeiyaZI]

Fixed.