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Discussion Starter #1
I was driving down the road the other day and all the sudden, no boost at all. You don't hear either turbo spool. Is there a solenoid or something that could have gone bad? There is no smoke or check engine light. Car still idles perfect and runs fine other than no boost. Thanks for the input.

Rob
 

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Boost

Rob:

This will not help but the same thing happened to me on my 1995 S4 about two months ago. I kept driving without the boost for about 3 miles and then it just went back on. The car has not done that again but it did rattle me a bit.

Hope someone with a lot more knowledge than me may know more.

Mike
 

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If the turbos aren't spooling in the first place, I'd start with the waste gate action since that is a likely reason for the turbos not doing anything. There are several reasons why the waste gate might not be functioning, including the most obvious of it being stuck (free movement check is part of regular maintenance). Another is a split in the vacuum line used to actuate it.

Knut


I was driving down the road the other day and all the sudden, no boost at all. You don't hear either turbo spool. Is there a solenoid or something that could have gone bad? There is no smoke or check engine light. Car still idles perfect and runs fine other than no boost. Thanks for the input.

Rob
 

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Discussion Starter #5
If the turbos aren't spooling in the first place, I'd start with the waste gate action since that is a likely reason for the turbos not doing anything. There are several reasons why the waste gate might not be functioning, including the most obvious of it being stuck (free movement check is part of regular maintenance). Another is a split in the vacuum line used to actuate it.

Knut
The wastegate moves freely. Will check vacuum line tonight.
 

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I'm no expert, but maybe check your temperature gauge. If your temp is too low (or high) I believe the ECU will cut boost. Just a guess though.
 

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Check the hoses and vac lines as mentioned. Another possibility is one (or both) of the turbos has "bitten the dust". Remove the inlet hose and spin the wheel. It should turn easily and not have any play in it or hit the housings. Do this with the motor off and cool. Pull the outlet hose on the air side and see if there is any oil on the inside of the hose. How many miles on the car? Check the throttle, when the pedal is all the way down you should have full opening on the throttle. Make sure the carpet or the floor mats are not interfering. As also mentioned, disconnect the linkage and make sure the wastegate moves freely on EACH turbo. Don't forget to hook it back up!
David Teitelbaum
 

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The wastegate moves freely. Will check vacuum line tonight.
If you'd like to work your way backward, then you might use a MightyVac brake bleeder to draw some vaccum on each waste gate actuator to ensure they operate before moving farther upstream and drawing a vacuum at the far end of the vacuum line to similarly verify operation. I like to start at the end and work my way upstream when debugging.

Knut
 

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It's not vacumn that opens the wastegate but pressure. Anyway that's not the point, if the lines to the wastegate are leaking or broken then the wastgates never open and you would have to much boost. Hard to imagine both turbos seizing at the same time, it could be a bad temp sensor (seperate from gauge) so the ECU would not allow more than the 5# mechanical boost level. What does the MAP sensor read via obd2 when under full throttle?
 

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You are correct, I would not expect both turbos to die at the same time. What is more likely is that one died and was not noticed but when the second one bit the dust now you notice because now you have nothing. Similar to when you lose headlights. Many people do not notice when one is burnt out but when they are BOTH gone it is impossible not to notice! Anyway, if he is having a problem with boost it can't hurt to at least look at them! I have found (through many years of experience) when you start looking, very often you stumble over the problem. My main question is, "How does he KNOW he is not getting boost?". There is no boost gauge. I suspect this has nothing to do with boost at all.
David Teitelbaum
 

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Hard to imagine both turbos seizing at the same time, it could be a bad temp sensor (seperate from gauge) so the ECU would not allow more than the 5# mechanical boost level. What does the MAP sensor read via obd2 when under full throttle?
This is what I was thinking with the temp gauge. But This brings up a good question... How does the computer limit boost?

If it just opens up the wastegates, then he might be able to just look under the car while revving the engine to see if the wastegates actuators are moving. If they are, then the turbo pressure is being dumped.

But if the ECU limits boost in some other fashion (like diverting the pressure elsewhere like a bov), then you wouldn't be able to monitor it that way.

David's logic is sound though that maybe one turbo was already dead and now the other has bit the dust. The V8 has so much power that if you are unfamiliar with its true performance it can be easy to think that a reduced output is normal. The same goes for the throttle cable adjustment - my full throttle was only at 82% and I never knew. But now that it's at 100% I know the difference!
 

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not sure about the V8, but on the SE and S4 models, there is a boost solenoid that controls the signal (boost pressure) to the wastegate actuator. if the fue blows to the solenoid, you still get some boost, but only whatever the actuator spring is set to. The V8 models don't have a boost gaguge on them.....are you sure you aren't getting any boost? I had the fuse blow on my boost control solenoid and the car felt alot weaker, but it still was registering about 7 psi on the stock gauge.

Look through your owners manual and find where the boost control solenoid fuse is located. On the V8, it is likely in the trunk fuse panel.

I still don't know why mine blew, but it has been fine ever since.
 

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The wastegates won't move when just reving the engine, it needs a load to generate exhaust flow to create boost of 5# to begin opening the wastegate.
 

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Discussion Starter #16
not sure about the V8, but on the SE and S4 models, there is a boost solenoid that controls the signal (boost pressure) to the wastegate actuator. if the fue blows to the solenoid, you still get some boost, but only whatever the actuator spring is set to. The V8 models don't have a boost gaguge on them.....are you sure you aren't getting any boost? I had the fuse blow on my boost control solenoid and the car felt alot weaker, but it still was registering about 7 psi on the stock gauge.

Look through your owners manual and find where the boost control solenoid fuse is located. On the V8, it is likely in the trunk fuse panel.

I still don't know why mine blew, but it has been fine ever since.
Does anyone know what fuse this is? It is something simple I can check tonight. That would be great if it was indeed a fuse. I put the car on a lift and didn't see any problems with the pipes, but there is one that goes under the intake manifold that I can not see the whole thing. It looks like it had been replaced one time as it is blue silicon.
 

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If you have an electrical problem with the boost you would see an error code and the MIL light would be on. If you are not getting boost the problem is likely to be mechanical. A leak in a hose, a bad turbo, a stuck wastegate, etc.
David Teitelbaum
 

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I find it odd that you have no boost at all...I would check for any possible torn or ripped open hoses. The wastegates would actually have to hang open on for you to have no boost at all or the actual plenum connectors off...or some other massive event like chewing up your blades. Even with a vacuum leak between the solinoid and wastegates or defective boost solenoid you will have some boost but then loose it at around 3000rpms as the waste gate springs are overcome by the exhaust pressure ...almost like a clutch slip or wheel spin. ( I had mine connected backwards so it was dumping boost 100% of the time by accident once )

The boost solenoid is nothing more then GM / Delphi EGR valve...can be had for 20 ~ 30 bucks. It works by the ECU turning it off when it wants to bleed or cut boost. when its on plenum pressure will travel through the solenoid and to the turbo wastegates holding them closed....when the computer activates the solenoid the pressure is bled off in a controlled vacuum leak at the valve the wastegate springs are allowed to be overcome by the exhaust pressure.

Obviously there is also the possibility of ECU failure but once again...you still have wastegate springs do some work
 
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