What does overboost cut off / over rev feel like?

[Paul93Lotus]

Posted here as well as the Yahoo board....

I have had a intermittent problem that I have been unable to track

down...

This problem only happens on full throttle. The revs will be

climbing and I will be at full boost and it will all of a sudden

feel like someone just turned the engine off. Now a couple of times

when this has happened I have not been looking at the tach or boost

gauge, and so I don't know what my rpm's were when it happened. The

last time was when a C6 vette pulled up next to me on the freeway.

As the hand of God pushed this new vette swiftly by me (and I heard

its supercharger whine / aftermarket exhaust growl...and probably

500+ HP)I knew I was in trouble, but kept my foot in it to

represent, and then the 100% loss of power abruptly occured (and the

end of me trying to keep up). I have noticed that the boost gauge

frequently reads 1 bar, and at times will read above to about 1.25

bar. Now I don't think I was over revving at any of these times(but

my eyes were more on the road and traffic), but wanted to be sure

the difference between overboost cut and over-rev cut off.

I looked at all the vacuum lines last night and everything looks ok

at the wastegate and boost control solenoid. I had never encountered

this problem before I powdercoated the plenum, chargecooler, and

intake. The whole intake assembly was removed along with gaskets

replaced, etc.I removed the throttle jack assembly, and then put a

vacuum cap at the throttle jack solenoid when I reassembled.

Anyone have this similar problem and figured it out? I hope to

Freescan it this weekend and datalog, hoping to catch this problem

in the act...but any insight would be greatly appreciated!

[Artie]

Sounds like you hit boost or fuel cut. Typically the ecm has a max measure of boost(in bar) or Revs. Overrev usually has a similar feel but you'll surely notice the RPM is too high. Fuel cut is usually a similar situation but it is designed to save the engine from damage when too much boost is seen, thus loss in power. The stock map sensor is a 1.25bar sensor IIRC, so it's possible you peaked to the max for a couple seconds and this triggered fuel/boost cut.

Artie

[Vulcan Grey]

Overboost cutoff definitely.

Check the wastegate control lines, then check your MAT reading in Freescan.

If the MAT is too high, then the ECU should limit boost, if it can't or if it builds too fast then it will do the overboost protection. This can also happen when the outside air temp is much lower than the last time you drove it, and the ECU will have to relearn (or be reset) for the new air temps.

I bet your chargecooler pump is shot.

Or a wastegate control line.

[Dermot]

Freescan logfile should reveal all...

Happy for you to send it to me over the weekend.

Dermot

[CNH]

If it feels like you are running out of fuel it is because the ECU cuts the fuel pumps on overboost.

Once the overboost conditions clears the fuel pump will cut back in. You can drive as long the turbo does not boost past 1 bar. That means the car will idle fine and cruise as long as the throttle plates do not open up to engage high boost.

The chargecooler pump failure will not cause this as the high temps will limit boost to about .65 bar.

Check the small hose to the wastegate capsule and wastegate itself as the overboost sensor will cut the fuel pumps. flow the hose as they have been known to crack and fall off on either end. The MAP could have gone bad but I would go for the hose to the capsule first.

[Gav]

Friend had this on a 92 car. Turned out the wastegate was seized and hence not controlling boost. The overboost cutoff was doing exactly as designed. As you described, the car ran fine unless you booted it and the full boost was applied.

After soaking the shalf in WD40 and working the shaft free, the car returned to normal. The shalf should move away from the control module then spring back when released.

HTH

Gav

[Vulcan Grey]

If it feels like you are running out of fuel it is because the ECU cuts the fuel pumps on overboost.

Once the overboost conditions clears the fuel pump will cut back in. You can drive as long the turbo does not boost past 1 bar. That means the car will idle fine and cruise as long as the throttle plates do not open up to engage high boost.

The chargecooler pump failure will not cause this as the high temps will limit boost to about .65 bar.

Check the small hose to the wastegate capsule and wastegate itself as the overboost sensor will cut the fuel pumps. flow the hose as they have been known to crack and fall off on either end. The MAP could have gone bad but I would go for the hose to the capsule first.

In the case of the chargecooler pump... if it is dead, and the turbo surge is fast enough, the ECU will go into overboost protection. If it's not able to reduce boost fast enough, which is often in some cases.

[ragingfool35]

mine does it all the time

i suspect a leak in the intake system

my bov spring was too weak

i replaced it with a stronger spring

and the symptoms are not as extreme now

if boost pressure leaks out (in the intake system) downstream

of the pressure hose to the wastegate,

the frequency valve is not fast enough to

compensate and overboost occurs

there are many potential causes of which this is one

[ozontour]

I had a similar issue last year, when my engine was only revving to 3000 revs and was boosting to over 1 bar.

It turned out to be a wastegate diaphragm. I replaced the wastegate with a local turbosmart streetgate wastegate for not a lot more than a new garret diaphragm and now I have reliabilty as well as a better contol of boost!

Hope this helps

Justin

[Paul93Lotus]

Wow, Thanks everyone for the quick replies. I will try to datalog the problem before I try a fix, as I don't want this to happen again (where it might be a more humble car than a supercharged C6) and know I have positively solved the problem.

After the data log, I will replace all waste gate associated hoses, double check my blow off valve and tubing, the coupler hoses, lube the waste gate shaft, etc. I now feel pretty confident I will find the issue!

[CNH]

In the case of the chargecooler pump... if it is dead, and the turbo surge is fast enough, the ECU will go into overboost protection. If it's not able to reduce boost fast enough, which is often in some cases.

Maybe it can happen on a rechipped car. Never seen fuel cutoff on an stock SE due to chargecooled pump failure. It will limit boost to the same boost levels of non charge cooled car. Even if the pump was working correctly it will only give 1+bar for a short time before settling down to .85. If one is experincing fuel pump cuttoff the problem it will most likely be in the wastegate operation. The ECU will 1st attempt to reduce overboost by opening up the wastegate if gate does not open up for some reason then the fuel pumps will cut out.