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Car jolts violently on boost?

ix9

By ix9,
in Engines & Forced Induction

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ix9 Grand Master

ix9

Posted
Posted

Guys,

A little bit of background: My boost is set to 10-11psi using a bleeder valve (Turbosmart) I have stock IC, ECU. I have an exhaust, POD, CAI, usual stuff....

I found that if I increase boost to say around 12-13psi, when under load at about 3000rpm, the car will jolt violently for like half a second, then resume accelerating.

I have since then wound the boost back down to 10-11psi and when in 5th gear, cruising under 3000rpm on the freeway, it's fine. As soon as I put my foot down, and boost climbs to 10psi.... at about 3000rpm the car will jolt violently then continue.

I've worked out a few things:

It's not detonation, as the boost _really_ isn't high enough to cause a problem.

I'm 90% sure it's not misfiring, as I've had that before with bad plugs, and is fixed. I have copper plugs gapped to 0.75

Similarity: I've had the exact same jolty feeling if I've got under a 1/4 tank and hammering the 'go' pedal around a corner, Obviously the fuel isn't getting in there somehow, but then again It's coz i'm low on fuel :P

One more thing:

When boost climbs to 13psi it SCREAMS (the noise I mean) gets really loud... very loud induction/turbo spool noise. Is this normal?

Does anyone have any ideas?

-Kym.

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Lozza150 Proficient

Lozza150

Posted
Posted

... maybe the fuel pump.... not enough juice to feed the cylinder's on strong boost.....

The turbosmart thingy could be the culprit..... throwing the "Spool up boost" past 12psi on "slam foot on floor" boost....

rough guess on my behalf

whatsisname Grand Master

whatsisname

Posted
Posted

I'd say you're hitting the AFM voltage sensed boost limit (usually ~12psi but it depends on air temp/density).

If you feed the power on even at 13psi you probably don't experience this jolting right? But if you nail the throttle is jolts.

This is due to the rush of intake air through the AFM as the the turbo spools up suddenly. This sudden rush of air causes the AFM voltage to exceed the AFM sensed boost limit. When you feed the throttle on the turbo doesn't spool up with such a big rush & the airflow through the AFM doesn't exceed its voltage limit.

The cooler the ambient air temp the lower the boost level that this will occur. Cool air being denser means more air through the AFM & higher voltage levels. A -2'c morning & don't be surprised if it cuts @ 10psi.

AndyMac Experienced

AndyMac

Posted
Posted

I agree with "whatisname". I'll be expirencing this morw with the cooler weather coming in soon.

The bleed vavle may not be the problem, I have a Turbo XS bleed vavle and have NEVER had a major spike. My car has been set to 12psi and never had a problem (I've removed the little rubber hose which goes to the boost sensor, so it can't read an.) over boost.)

A few more things..how are you reading boost ? if its a aftermarket gauge where is tapped into ? do you have a atmo. BOV...is it dirty or worn and could be venting under boost load ?

After having a look at those things, I'd be cleaning your AFM (carfully with silicon spray) and taking the intake pipes off and spraying some carby cleaner down the intake manifold...

after that, I'd considar the boost cut/fuel pickup problem..

Cheers

ix9 Grand Master

ix9

Posted
  • Author
Posted

Guys, thanks for the replies..

whatsisname: Yeah, that's right, if i feed it gradually it's fine. If i nail it, it jolts :P

PeakRPM: I'm reading boost where the stock BOV was connected. This is now connected to another vacuum hose that used to goto the Blitz bov (disconnected properly)

Metblue: Well, I know what the POD filter sounds like... kind of a "hollow" sound if that makes any sense.. but this sounds like air rushing out of a full compressor.. it's REAL LOUD... hmmm makes me think.

Gradenko Mentor

Gradenko

Posted
Posted

Strich9ine, more than likely it is an AFM voltage cut. Had the same experience when setting up my bleeder. On top of whatsisname comments, also be weary of the accuracy of your boost gauge. I was hitting the cut at an indicated 10psi, which was way off.

Originally posted by SHUTO-BOY

im gonna ditch my bleeder valve,i hate the jolting(its more like "pushing back") get an electronic boost controller. A fuel cut defender, or even better ecu and delete the airflow meter

EBC may help hold boost better but it won't get rid of the AFM cut. Fuel cut defender will do that, but its fairly dangerous without aftermarket tuning to match. The AFM cut is there for a reason and getting rid of it while running stock fuel maps is a recipe for disaster. Personaly, I don't see any reason for deleting the AFM. Its a great measuring device (better than using MAP for road driving, IMHO) and isn't a huge restriciton like a flap AFM or Karman Vortex MAF. Only downside is really the reliability of the AFM.

Originally posted by PeakRPM

After having a look at those things, I'd be cleaning your AFM (carfully with silicon spray)

Before anyone tries this and gets into trouble, WD40 and AFMs do not mix well! Use electronic contact cleaner (1st choice) or carby cleaner (2nd choice).

Hicks Mentor

Hicks

Posted
Posted

Kim, Its your boost cut. Boost cut is set at about 14.5 pounds, but what is happening is that your putting your foot down in too high a gear without enough revs. This puts a lot of pressure on the engine and the boost goes straight up extremly fast!

I bet you take off from the lights, red line first, red line second, redline third no worries. But say cruising in fourth at 60, put your foot down and it does it. Always drop back a gear or two.

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