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1996 Suburban no boost?!?

S

Shawnr001

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Location
Colorado Springs
Help!

I have no boost pressure and I can't figure out why the heck why.
Here is what I have done so far to diagnose.

Removed the vaccum pump and did the turbo master trick. This really didn't do anything to make note of...
Second, I bypassed the muffler and soot can and ran open pipe (that was fun around town :eek:) Just to see if I had anything clogged down stream in the exhaust. This helped slightly.
Third, I hard wired the wastegate in the closed position and ran it open exhaust around town. STILL hardly got 1psi boost even when pulling up a hill pedal to the metal. It was smoking, but not huge. Typical amount when you see a stock diesel powering up an onramp to the highway. But hardly any power... temp got hot quick though as you could imagine...
To note however, at this point I was able to hear the turbo spool for the first time in a long time, that was cool to hear...
Forth, I did the brutal task of taking the turbo out to inspect it. It looks perfect, turns freely, blades look perfect both intake and exhaust side, wastegate looks to be closing and opening completely smooth and sealing well, bearings are perfectly tight, etc... SO....WHAT THE HECK TO DO NOW!?!

In the morning I'm going to see if there are any air leaks in the intake manifold. Since I still have the turbo off I will block off the exhaust manifold and then apply compressed air into the intake tube that comes from the turbo and see if I can find any leaks.

Problem is I have in the back of my mind that even if there is an air leak, even a big one, I should STILL see more than 1psi boost...

PLEASE if you have any ideas what is going on here let me know!

Also note. It starts easy, runs smooth, and burns no oil. I have the latest greatest PMD and it relocated to the bumper.
The Suburban has 267k miles, I bought it at 160k 8 years ago. I have never serviced the injector pump or injectors. I have no idea if these were serviced before I bought it. I watch the fuel mileage on every tank it almost always gets 19.6 MPG.
In the past few months I have had the PO236 (or maybe it was 238), anyway I looked it up and it was turbo inefficiency or low boost. I got the SES light temporarily while cruising down the highway.

Thanks!
 
X2 Exhaust leaks will kill boost... Test the boost gauge. Have you removed the snorkel from the fender? Air filter is good/clean?
 
Hi Shawn! Welcome to the Truck Stop! Take a few minutes to fill out your signature (click the link in my signature, it'll take you to yours) and tell us about your truck.

x3 on those suggestions... I think you are into checking the complex stuff when there is still a lot of simple stuff left to do...

Getting rid of the vacuum won't hurt anything, but you might have checked to make sure the issue was there... you could have left everything alone and just put a tarp strap or spring on the wastegate arm. If your turbo didn't come back on, you would know that the vacuum wasn't the issue. From there, vacuum testing at various points would let you isolate the culprit.

So, theory: For the turbo to work right, everything has to work right.

1] First, make sure the gauge (and the line from the plenum) works.
- Did you *used to have* boost pressure?
- Did it show on *that* gauge?
If your gauge assembly isn't working, you're busy fixing the wrong problem.

2] The turbo won't spin (and whistle) unless it can get air in.
- Restrictions in intake airflow will prevent the turbo from working. (A snorkel, a collapsed filter, etc)
- A clogged airflow will silence a turbo (dirty filters hurt)
I've seen the cap off a cheap cone filter get into the turbo air inlet and choke things right off.

3] The turbo won't spin (and whistle) unless it can get air out.
- A restricted plenum or webbing can mess up turbo function
- A messed-up EGR or mystery gasket or other air leak can mess up 'apparent' turbo function
When the turbo --> engine path has leaks in it, pressure can't build.

4] The turbo won't spin (and whistle) unless it is driven by exhaust gasses.
- Any crossover leaks will kill it. (check the manifold connections)
- Crossover restrictions will kill it.
GM used double-wall pipe in some years and I've seen that collapse without anything being noticeable on the outside. (Take it off and roll a golf ball through it.)

5] The turbo needs to eject exhaust efficiently in order to work.
- The soot trap or muffler can clog off and the resultant backpressure can stop your engine
- the stock GM downpipe is pretty restrictive.
Sounds like you checked that, but we don't know where you dismantled things. If you left the stock downpipe bolted to the turbo (likely) and the downpipe is the issue, being loud wouldn't have meant the exhaust was free-flowing.


Every test you do should be designed to isolate and test something. Passing each test should rule something out. The trick is to diagnose the issue by logic, not by replacing the truck. :D
 
Last edited:
Thanks so much for the suggestions!

I don't think the turbo has ever worked right since I've owned it. The only time the Suburban really has any noticeable power is in the winter and that is the only time in the past that I have ever heard the turbo whistle and the whistle was only on occasion. For regular use and light towing the power has been ample, but not amazing by any means.

Before dismantling the vacuum pump system I did confirm that it was producing almost no vacuum first. Plus the bearings were going out. It's been out for some time as the pulley on the vacuum pump wiggles in and out and would eat the front edge of the belt. So two reasons why I needed to loose the vacuum pump.

I tested the boost gauge with a bicycle pump, gauges matched pressure so the gauge is good. Also in case your wondering I have the 1/8 NPT fitting to the gauge placed into the back side and middle of the pipe that leads from turbo to intake manifold. I have never run a boost gauge before, I installed this to diagnose the low boost issue.

Note, that in a previous test I removed the temp sensor from the manifold and started the engine with the wastegate wired in the closed position. I would expect air to be pushing with good force from the 1/2" NPT hole. But no. The pressure is basically equalized where there is no vacuum or pressure. I can place my thumb over the hole and it's difficult to tell if it's really sucking or blowing.

Also note that I am doing all these test with the intake completely off, the front of the turbo is open to the air. (don't warn me against this, I'm well aware of the hazard)

The engine also has no EGR.

The down pipe is free from any obstruction. There is no caking of soot anywhere in the system, only a light dust that can easily be rubbed off with a finger to expose metal.

This morning I blocked off the turbo pipe to the intake and added compressed air and watched the boost gauge. I brought the manifold up to about 5 psi and then let it bleed down not to exceed 5psi. When I reach 5 psi on compressed air it takes about 2 seconds for the pressure to bleed down to zero. I don't hear any leaks anywhere around the intake manifold. Most of the air getting out is from around where I blocked the turbo pipe. It takes a very small volume of air to get up to 5psi too. Which indicates to me that if there is a leak it is very very small if any.

I also pressure tested the exhaust manifold. I did this by putting a plate over the hole where the turbo bolts on and forcing air compressed air into the exhaust in the same method as the intake... There may be a leak as it bleeds down very rapidly, but any leak is not evident by any soot marks anywhere on the exhaust manifolds/crossover pipe etc...

The next thing I'm going to do is take the crossover pipe off and see if there is any obstruction. I think I am down to looking for obstructions at this point. I'm also going to put the turbo back on and start going back together. I'm going to keep a careful eye out for leaks.

Any other ideas PLEASE let me know! Thanks Guys!
 
Welcome to the Truck Stop. I'm chasing a leak too so I feel your pain as I'm about to pull the manifolds off the block to check for evidence of leaks at the mating surfaces.

Very thorough testing on your part though, congrats.
Check the cross over / exhaust manifold junctures for leaks; the doughnuts may have let go.
Inspect crossover surface for soot as it may have pin-holed as well. If it is an OEM crossover you may have the classic collapsed innards syndrome (any excuse is a good one for an aftermarket replacement).
As you have taken the turbo off before, did you note any soot on the turbo to exhaust manifold mating surfaces extending all the way to outer edges?
Any evidence of oil escaping from the silicon hose seals at the turbo to upper intake (band clamps); between the upper and lower intake mating surfaces (gasket); bolts holding upper intake down (17psi torque value); around the boost sensor seal (boot old, bolts tight)? I know you've pressure tested the set up but oil at these points are good indicators you have a pressure leak during operation.
Hang in there, you'll sort it out.
 
Checked and double checked. Thank you.

FYI if you have a stock crossover find something smaller than a golf ball to check for obstructions! I got a golfball stuck for a bit... THANKS JiFaire for that one! On that note,this is a good way to illustrate how restrictive the stock crossover pipe actually is! The bends are SMALLER than the diameter of a golf ball THAT IS BAD. DEFINATLY time for an aftermarket crossover.

Right now I have ruled out that I have no leaks in the intake or exhaust.

I did another test though. I removed the silicone hose that joins the turbo to the intake manifold. I did this to see how much excess air was actually coming out of the turbo, I figured that with this 3/8" air gap I could feel the pressure building while I had my wife rev up the engine. Again with the wastegate wired into the closed position I had her slowly bring the engine speed up to 1,500RPM in park, then again to 1,200 RPM in drive while she stood on the break. There was evident surplus air coming out of the gap that could build a little boost, but I wasn't impressed... I could clamp my hand around the 3/8 gap and there just wasn't much air pressure pushing past my hand. Even 1-2 psi of boost at high volume would give a little push back I would think?

So here is my questions now.

Is it possible that visually a turbo can look totally fine, but is just tired and simply can't build adequate boost anymore?

Second, can something else be going on to create the illusion of low boost pressure? For example if I'm just not getting the exhaust gas pressure from the cylinders in the first place the turbo won't run right. This may be a stretch, given that it starts easy, runs around town just fine, blows black smoke, but not tons... fairly normal I think...

I'm open for other suggestions. I'm really trying to reach for answers here! Thank you all again!
 
Also, what brand of crossover would you recommend? I'm not going racing or anything, I just need a good reliable vehicle to tow a camper through the mountains to take the family camping and dirt biking.
 
Shawnr001 said:
Checked and double checked. Thank you.

FYI if you have a stock crossover find something smaller than a golf ball to check for obstructions! I got a golfball stuck for a bit... THANKS JiFaire for that one! On that note,this is a good way to illustrate how restrictive the stock crossover pipe actually is! The bends are SMALLER than the diameter of a golf ball THAT IS BAD. DEFINATLY time for an aftermarket crossover.

Right now I have ruled out that I have no leaks in the intake or exhaust.

I did another test though. I removed the silicone hose that joins the turbo to the intake manifold. I did this to see how much excess air was actually coming out of the turbo, I figured that with this 3/8" air gap I could feel the pressure building while I had my wife rev up the engine. Again with the wastegate wired into the closed position I had her slowly bring the engine speed up to 1,500RPM in park, then again to 1,200 RPM in drive while she stood on the break. There was evident surplus air coming out of the gap that could build a little boost, but I wasn't impressed... I could clamp my hand around the 3/8 gap and there just wasn't much air pressure pushing past my hand. Even 1-2 psi of boost at high volume would give a little push back I would think?

So here is my questions now.

Is it possible that visually a turbo can look totally fine, but is just tired and simply can't build adequate boost anymore?

Second, can something else be going on to create the illusion of low boost pressure? For example if I'm just not getting the exhaust gas pressure from the cylinders in the first place the turbo won't run right. This may be a stretch, given that it starts easy, runs around town just fine, blows black smoke, but not tons... fairly normal I think...

I'm open for other suggestions. I'm really trying to reach for answers here! Thank you all again!
Click to expand...

Yes on the turbo performance. If it gets sloppy and the turbine grinds into the housing, the clearance increases and boost decreases, but it's unlikely to go to zero.

Pull the intake boot and observe the turbo while someone is reving it.

Your descriptions of its performance are close to normal. If there was no boost, it would be a dog and blowing a ton of black smoke, especially when towing.
 
Also note that even with my wife holding the engine speed at around 1,500PRM in park I actuated the wastegate linkage by hand expecting a little resistance from the exhaust gases, but there is hardly any. I've seen youtube videos from guys putting cameras under their hoods to show the action of the wastegate and spring on the turbo master, but with mine there is no way there is any pressure to move even the lightest turbo master spring...
 
I have chinese knockoff gm8(turbochargerpros) that I bought thinking it was a Borg Warner. I payed 6 for it , but would sell it really cheap if it would help you out. I never used it as I got an ATT instead. If you need a turbo I would suggest getting the ATT. If you are tight for cash you will like my price od 2 bills plus ride. Again your best bet would be ATT over a gm8
 
Shawnr001 said:
Also note that even with my wife holding the engine speed at around 1,500PRM in park I actuated the wastegate linkage by hand expecting a little resistance from the exhaust gases, but there is hardly any. I've seen youtube videos from guys putting cameras under their hoods to show the action of the wastegate and spring on the turbo master, but with mine there is no way there is any pressure to move even the lightest turbo master spring...
Click to expand...

Leak in the crossover, or the turbo is bad. Pull the boot and confirm that it's spinning with the engine running.
 
Just a side note, but can you remove the exhaust housing from the turbo and check for obstructions? When I first got my ATT I was having a real hard time spooling it. Fully maxed injection pump netted me 2psi at 3800rpm. I sent it to Slim and he had found that a very very small rock had somehow become wedged in the exhaust housing pre-install and had hampered flow that bad. Once the rock was removed, things were golden. just something to check if you have the time
 
Shawnr001 said:
Checked and double checked. Thank you.

Is it possible that visually a turbo can look totally fine, but is just tired and simply can't build adequate boost anymore? YES

Second, can something else be going on to create the illusion of low boost pressure? For example if I'm just not getting the exhaust gas pressure from the cylinders in the first place the turbo won't run right. This may be a stretch, given that it starts easy, runs around town just fine, blows black smoke, but not tons... fairly normal I think...

I'm open for other suggestions. I'm really trying to reach for answers here! Thank you all again!
Click to expand...

I think you've ruled out exhaust leakage as an issue since you've not discovered any soot/leakage at the known points of failure. That said, if you can't get an golf ball through the crossover...

I'd swap in a new crossover and doughnuts before I condemned the turbo. I understand that the turbo's can go bad even if it the blades on the exhaust side loosing their original shape (edges and surface worn over time from heat and soot etc.) and, therefor not performing as designed.

I have 635's old GM-? (3 or 4, I can't remember) that could be shipped to you for trial if you want to go that route. It isn't mine otherwise I'd say keep it cause it's just taking up space around here. Then again, it may be better to pick up Dave's CKO GM-8 given the cost of shipping one back and forth even if it is only between states.
 
Did anybody mention that little device that controls the Vac to the waste gate, forgot what it's called, anyway, that went out on my 3500, cheap and easy fix.
 
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