Constant Overboost

funnyman06 · **Joined:** Sun Jul 06, 2014 7:46 pm **Posts:** 180

Hey guys,

So I've been driving my jeep around alot for work and Ive noticed that I quite often get a CEL for overboost, but it typically only happens during 1 of 2 scenarios. Doing 70mph up this one good size grade and by the top ill have a CEL, this is cruising around 2000 rpms. The other time is when I'm in 3rd gear on a freeway onramp doing about 2500 rpms with part throttle. Pretty much guaranteed to trip a CEL. I can floor it to 3K and over, like im passing on the highway, with no issues, however its under part throttle acceleration that screws me. Anyone else have this issue?

Mike92104 · **Posted:** Mon May 18, 2015 7:45 pm

There's been 1 or 2 reports of boost issues. I would think the first step for you is to se if the vanes are stuck or sticking.

diesel_guy86 · **Posted:** Tue May 19, 2015 6:48 pm

If it was UNDER boost it could be several things, but overboost HAS to be vane related, whether it's the vanes themselves, or some part of the vacuum/electrical acuation system.

First step is to pull the vacuum line that goes to the turbo and check to see that the acuator rod drops a half inch or so. That's the easiest test to do and tells you if it's in the turbo, or the system.

flash7210 · **Posted:** Tue May 19, 2015 8:48 pm

Could be the map sensor.

diesel_guy86 · **Posted:** Tue May 19, 2015 9:01 pm

I thought of that, but I wouldn't think it's behaviour would be that predictable. Could be wrong though. The vane test is still easy, and free, vs buying another sensor.

funnyman06 · **Joined:** Sun Jul 06, 2014 7:46 pm **Posts:** 180

Map sensor is brand new, that was my first thought. Ill play with the turbo, see what I can come up with.

geordi · **Posted:** Wed May 20, 2015 12:09 pm

A new sensor can be had for $25 from IDparts and is a perfect replacement option. With a pricepoint like that, it is hardly worth retaining a buggy sensor if there is even a question.

That said, has this problem developed over time or suddenly? Overboosting in my experience came with a bad VNT section on a brand new turbo, and until I replaced the back half of the turbo with another VNT section, the problem didn't go away. Overboosting is VERY VERY BAD for this turbo, as it already operates too close to its own operational safety limits. Spikes above 26psi can bring the turbo shaft speed too close to the failure point, which could lead to catastrophic failure and potentially the loss of the entire engine.

This is fixable, but I think most methods of repair will involve removal of the turbo from the engine. Either for cleaning with Easy-off or a full replacement of the back half with another unit. Luckily for you, the back half is not damaged when the turbo fails, so maybe one of the other members who has recently suffered a shaft or compressor failure could sell you his back section.

NEVER DISASSEMBLE THE CENTER CARTRIDGE with the two wheels, at least if you have any plans to re-use it ever again. That is the precision part, and it is very precisely balanced. The center cartridge is not specific to the turbo, you can replace the front and back housings (and the VNT section) without any problem. This is what I did when repairing the defective VNT section on my turbo.

funnyman06 · **Joined:** Sun Jul 06, 2014 7:46 pm **Posts:** 180

Exactly the reason why I replaced the sensor first. Figured the old one would be covered in crap too, it wasnt that bad though.

I should probably talk to keith about this but I would like to understand more on the control method of the turbo and what exactly the overboost CEL means. Does it mean that the current boost is greater than the target boost or is there a fixed setpoint that creates a CEL when crossed? I certainly believe the turbo is capable of overboosting under part throttle with the variable vanes, but find it hard to believe that I could be overboosting at low RPM part throttle and be fine when I go WOT. So I guess im really just wondering if with the weeks elbow and reduced restriction on the inlet side of the engine am I just flowing more air than expected and falling off of the table?

Also to answer you question, its been popping this code ever since I got the jeep back from its rebuild. It wouldn't surprise me if the vanes were sticking, the jeep was ingesting a lot of coolant and that could have had its way with the exhaust section of the turbo.

Thanks!

geordi · **Posted:** Wed May 20, 2015 1:32 pm

funnyman06 wrote:

Exactly the reason why I replaced the sensor first. Figured the old one would be covered in crap too, it wasnt that bad though.

I should probably talk to keith about this but I would like to understand more on the control method of the turbo and what exactly the overboost CEL means. Does it mean that the current boost is greater than the target boost or is there a fixed setpoint that creates a CEL when crossed? I certainly believe the turbo is capable of overboosting under part throttle with the variable vanes, but find it hard to believe that I could be overboosting at low RPM part throttle and be fine when I go WOT. So I guess im really just wondering if with the weeks elbow and reduced restriction on the inlet side of the engine am I just flowing more air than expected and falling off of the table?

Also to answer you question, its been popping this code ever since I got the jeep back from its rebuild. It wouldn't surprise me if the vanes were sticking, the jeep was ingesting a lot of coolant and that could have had its way with the exhaust section of the turbo.

Thanks!

Keith could certainly provide more insight, but from what I remember about my overboosting incidents - it is a combination of both. The computer sees more than it is requesting (which should result in a reduction of the vacuum pull on the vac motor) and either the turbo doesn't react fast enough, OR that it is still capable of exceeding 26psi even at part throttle.

That brings up a thought as I was typing it - Have you checked the vacuum filter on the modulator? That little paper filter is what allows normal air in to reduce the control vacuum when the computer commands less boost. If the vac isn't reduced fast enough, that could easily lead to overboost codes, while not being above 26psi - you would just be exceeding the setpoint that the computer was requesting, and annoying it that way. It could also be the modulator itself, but I think that is less likely unless it is somehow still partially functional but "slow" in reacting or sealing one side or the other.

papaindigo · **Posted:** Wed May 20, 2015 6:02 pm

I'm speculating a bit but not that much. A variable vane turbo varies the amount of boost by moving the vanes either by using a vacuum source (OEM turbo setup) or electric driver (GDE Stage II turbo). I do not know if the "speed" of the turbo "fan" varies all that much but in any case it's the vanes that do most of the work as far as I know. As geordi says check that filter but also check the variable vane actuator rod as also mentioned. If the vanes are stuck or tending to stick in a "high" boost position then I would not be too surprised to see an overboost code and lower rpms and not at higher ones simply because the computer system is "seeing" more boost than expected at lower rpms as a result of vane position. The system is not sophisticated enough to report that the overboost is caused by sticky vanes so it just says overboost.

geordi · **Posted:** Wed May 20, 2015 8:24 pm

The movement of the vanes directly sends exhaust pressure into the turbine wheel of the turbo shaft, massively increasing its rotational speed. This increased rotation is a linear ratio to the developed boost from the compressor side. At 26psi, the center shaft is at or above 120,000 rpm. I don't know exactly what the maximum limit is for this turbo, but the factory computer limits are very close to that.

Other than that difference in his description, Papaindigo has it right on.

mass-hole · **Posted:** Thu May 21, 2015 1:41 pm

Mine does it if I am really beating on it, IE towing my buddies 5000 lb wakeboard boat at 70mph up a hill, but I have never had it do that at partial throttle.

From when I was digging around in the tunes, I never was able to find a single value boost limit which is what they call the hard cutoff in the Bosch ECU's. This doesnt mean it isnt there, but it wasnt where it should have been according to the tutorials I was using to try to find the maps in the tune.

funnyman06 · **Joined:** Sun Jul 06, 2014 7:46 pm **Posts:** 180

Thanks for all the information. Ill tear into it a little more and see what I can come up with. Also I just found that the plug my mechanic made for the EGR tube on the exhaust manifold i leaking by, so im gonna order the plug from weeks and see if that helps. I could be leaking exhaust sometimes but not always, sending the turbo into a hissy?

Thanks again!

Constant Overboost

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