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Robt 394 09-25-2011 11:07 AM

Boost No Boost odd pressure?
I have an all stock 89 930 that is a low miles car. I have been having boost issues for a while now, and finally got around to installing a manual gauge from NH speedometer that taps into the brake booster. Here is what is happening.

On start up the gauge drops to -0.5 bar. Assume this is normal as it is pulling a vacuum. But if not let me know. After warming up the car it will build boost up to 0.6 bar. Nothing over, and I have the stock spring that I believe should allow 0.8 bar. If you cruise for a bit, say 15 minutes, you can no longer get any boost at all. Gauge will move from -0.5 to 0. I am ruling out heat soak as weather has been perfect with plenty of cool outside air.

If you pull over, turn off car and wait 5-10 mins and then get back on it you get 0.6 bar again. All is normal. But that works for one pull, two at most. Quickly you go back to zero boost. Stop, restart and then once again you get boost for one pull.

Whatever it is, it is doing two things. Limiting total boost and then also it seems to "stick" when driving. But when you turn off electrics it releases. And then of course sticks again to prohibit boost.

So, any ideas? Would you start with the control valve and boost pressure switch? Replace these, test them, other suggestions? Although the car has low miles and electrics are in good shape these parts are original and over 22 years old at this point.

DSPTurtle 09-25-2011 11:50 AM

Boost is not electronic... it is strictly mechanical if your car is stock. If you are blowing exhaust you are spinning the turbo. 0.6 might be all that your old waste gate spring will hold. 0.7 is the actually reading I was getting out of my stock spring, so 0.6 is not that far off.
Does the car feel like it is not making boost when you drive it? You should be able to tell from your butt dyno.
The gauge should show vacuum at idle so that is correct. Are you only having a gauge issue or is the issue with the drivability of the car?

Robt 394 09-25-2011 12:07 PM

Butt dyno says no boost and it takes forever to get to redline. Definitely a mechanical issue and not a gauge issue. At idle the gauge is at -0.5 bar. Does that seem strange? I can rationalize it, but sure seems like it ought to be closer to zero. When you give it some gas at low RPMs the gauge will approach zero, but then settles back down to negative boost.

DSPTurtle 09-25-2011 12:34 PM

Yes, negative pressure is the vacuum that makes your brakes work. That is not your problem. Were you having boost issues before you installed the gauge? Or is this new related to your install?

Robt 394 09-25-2011 12:38 PM

I was having issues before the gauge was installed. I mainly put in the gauge to confirm what I was feeling and also what the factory gauge was telling me. What is odd is the "on" then "off" nature of the problem. Which is why I am wondering if one of the control valves is sticking for some reason but resets when you turn off the car.

DSPTurtle 09-25-2011 02:13 PM

That is an interesting problem... any smoking from the tailpipe? Have you had the intercooler off the car recently? Perhaps your intercooler seals are ripped and when things heat up they are leaking boost in a big way. Do you hear any whistling when you are under boost and everything is working?

Robt 394 09-26-2011 04:02 AM

Thanks for the help. I read as many posts as I could find. No black smoke and IC seems to be sealed and in good shape. No whistling. I will verify the IC again this weekend, but doubtful that is the issue.

I am thinking it could be the boost pressure switch: 930-606-101-00-M100 or
The change over valve: 911-618-123-00

Any thoughts out there? My sense is on start up it works fine. But something is causing the system to close and prevent boost after the first couple of pulls. That is why a switch seems like it could be the issue.

mark houghton 09-26-2011 04:47 AM

As DSPTurtle said, boost is not electronic. The switch you're referring to is an over-boost protection switch that shuts down your fuel pumps in the event of an overboost situtation. It has nothing to do with building boost or the lack thereof. Nor does the changeover valve you're referring to have anything to do with boost.

If you turbo is creating sufficient boost, and if it were to leak out before getting to the intake, you would see a pig rich condition (poor running and black smoke). But it appears you've ruled out any plumbing leaks in the IC circuits. Other possibiliites is that your wastegate is stuck or sticking open, letting the exhaust gasses bypass the turbo to where it won't spin at full speed. Check if you have any exhaust exiting the WG dump pipe at idle (of course after your no-boost symptoms appear) to see if perhaps the WG is stuck open.

Other thoughts are your boost recirculation valve assembly (housed in the intermediate intake manifold atop the main manifold). Perhaps it's stuck open as well, but I don't know of anyone that's actually seen that happen.

Finally, your turbo may not be spinning freely after it gets good and warm. Check radial play in the shaft...the impeller may be contacting the side of the turbo (not good for obvious reasons).

DSPTurtle 09-26-2011 04:54 AM

Wow, Mark... you jumped right into the hard stuff. :)
If you don't have a waste gate muffler, finding a sticking waste gate would be easy. Otherwise there are 4 really difficult to remove nuts holding on the baby muffler. If you take the baby muffler off, you will be able to hear the difference in exhaust note when it is open. This would be the easiest for you to check and repair if necessary.
As far as spinning the turbo, this requires that you remove the rear bumper to get muffler and tailpipes off. Unless someone can give you a better recommendation on how to get to that 15mm nut at the top of the four mounting studs :(

Robt 394 09-26-2011 05:08 AM

Thanks Mark. I did check the WG at idle, and no exhaust. But of course that was just after starting it which is when we know it builds boost. I will check again. I was planning a rebuild of the WG anyway, and might do that in the next couple of weeks.

This winter I was planning to look at a Fabspeed muffler. Sounds like I should check the turbo at that time, and possibly send to Turbokraft for a rebuild. Regardless, this is not going to be a 2 hour project. Hmm, might be a winter project and just enjoy the fall for now.

infraredcalvin 09-26-2011 06:20 AM

Is your car exhibiting any other symptoms under the no boost issue? Runs hot, idles poorly, etc??? Could it be a fuel related problem? Perhaps it's one of your fuel pumps on it's way out, causing an extreme lean condition. Might explain why it seems electrical in nature, off, allows it to cool just enough to kick back on, then when hot again it shuts down.

mark houghton 09-26-2011 10:14 AM


Originally Posted by DSPTurtle (Post 6275454)
Wow, Mark... you jumped right into the hard stuff. :)
If (

Yeah, I tend to do that don't I? Just doing a mind-dump....:)

One more comment on sticking waste gates: This wouldn't rear it's ugly head as a cause unless Robt134 was first pushing max. boost enough to open the WG. Once opened, though, if the valve guide or seat were shot or seriously corroded, it might hang open even after the boost drops off. But as stated, that's the easier piece of the puzzle to investigate.

Robt 394 09-26-2011 10:24 AM

Thanks guys for the advice. In case you cannot tell I know very little about turbocharging but it looks like I am about to learn.

As for pushing max boost, I simply mash the pedal in first, shift to second and mash again. It is a great feeling when it works. If that causes max boost then perhaps the WG is opening. The whole WG device looks rather corroded as if it has never been cleaned. Fortunately it is easy to get to.

As for other symptoms during normal driving. Idle is perfect, WUR actually works well, and while it takes a while to spin up the RPMs when turbo does not work it will rev up to redline. I am not ruling out a fuel pump as they too are 22 years old. Perhaps I will simply replace these as part of the 30K mile maintenance. Sounds like I have a rebuild bill coming my way.

Hopefully I can handle most of the labor myself.

mark houghton 09-26-2011 12:19 PM


Originally Posted by Robt 394 (Post 6276052)
Thanks guys for the advice. In case you cannot tell I know very little about turbocharging but it looks like I am about to learn.

As for pushing max boost, I simply mash the pedal in first, shift to second and mash again. It is a great feeling when it works. If that causes max boost then perhaps the WG is opening.

If you're pinned to your seat and your cheeks are flapping back against your ears and you're wearing this grimmace of joy and fear on your face, then you've got boost happening when mashing the first two gears and you should be able to hear a distinct change in the exhaust note when the WG opens.

Don't sweat the small stuff learning about these beasties. Everyone has to start somewhere, and I dare say that every one of us are still learning.

Here's something more to consider: Your stock '89 - I presume - still has the catalytic converter, correct? Get rid of that thing ASAP. It's probably partially plugged up anyway and will have a marked affect on turbo spooling. You can either install a new muffler system, or keep your stock and install a bypass pipe in place of the cat.

Robt 394 09-26-2011 01:16 PM

The silly grin and cheeks flapping in the wind I know all about. Love that feeling. It is the ping I have never noticed as the exhaust is not that loud or as intoxicating as I would like. I will pay more attention this weekend.

This is a great car to tool around in as it is mellow when you need it to be, looks fantastic, and has all the HP and Torque you need for street driving. I really enjoy the car and plan on keeping it. Would like to keep mods to a minimum, but ...

... that cat has to go. Hence the debate begins. I would like to keep it mostly stock so the bypass has come to mind. But if you are changing one part, why not go the whole way and put on a better muffler / bypass system as it just bolts on. We all know how slippery that slope can be. The long neck IC looks like a great option as well as it simply "bolts on". Easy to take it back to stock. I don't plan on selling the car anytime soon and as long as I can easily put it back the way it was then why not add to the fun while I am driving it.

The thread has at least pointed me in the right direction on what to do. I was planning on rebuilding the WG as I just suspected it was not working 100% as it should. I can pull it and the turbo and rebuild if needed when I do the 30k mile service. Now I just need to find the time.

JFairman 09-26-2011 02:07 PM

If the car has only 30,000 miles on it the fuel pumps are fine. They are usually good for around 150,000-200,000 miles.

Robt 394 10-01-2011 02:51 PM

Here is the latest. Last week the ambient temp was 65 to 70. By no means hot, but not cold. Today it was 45 to 50 all day. The car consistently builds .65 bar in 2nd and 3rd, but will build .7 to .75 in 4th. It did this all day long and I could not replicate a no boost situation. What a difference, and the car is much more fun to drive.

I am guessing the WG is OK based on this behavior but by no means am I ruling a rebuild out. It is possible the Intercooler is the problem and that on a cool day the issue does not occur as the ambient air is cool enough to mix with hot air in the engine compartment and the end result is still cool air IF for some reason I have a leak or bad seal.

It is interesting that the pressure varies by the gear. All I can guess at is when the car is moving faster the air is cooler and cleaner making the intercooler much more efficient.

mark houghton 10-01-2011 05:02 PM

It's just a physics thing, Rob. Though it's been about 35 years since my last physics class, I think it's all about density. Cold air = more dense = more mass. Your turbo will move the same volume of air, but since it's more dense at colder temperatures you're actually cramming more air into the intake. That's about the most butchered explanation I can give. My car positively roars when it's a cool 50 degrees (or less) outside.

Cold air helps, but you've still got a boost problem. Cold outside or not, you should have no problem reaching max boost.

DSPTurtle 10-01-2011 06:29 PM

Come on Mark... you usually give better advice than that. LOL
Robt, when is the last time you changed your IC seals? Do you have a stock BOV? There are only a handful of reasons you are not getting boost. There are only so many components that have positive pressure air blowing through them. Easy peasy to figure out if you take it slow.

mark houghton 10-01-2011 07:10 PM


Originally Posted by Robt 394 (Post 6276052)
Idle is perfect, WUR actually works well

Nothing better to do tonight but lurk around here with a glass of wine in one hand and the mouse in the other.

So, how do you know your WUR works well when you're attempting full boost? If you don't have a serious boost leak (and it doesn't sound as though you do by your descriptions), or if your turbo isn't about to self-destruct due to bad bearings or excessive runnout, and if you haven't neglected to clean your air cleaner in the last few years, then fueling could be part of the problem. Start with the things you can see/touch/feel, looking for the obvious...before you let people like me send you toward all the possible doom and gloom stuff. It's usually the least complex things that are causing the problems with these cars.

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