No turbo boost at times.

[arveetek]

After installing my turbo boost guage on the Tahoe, I've been able to monitor what's really going on under the hood. Most of the time, the boost seems normal, with a max of 7 psi under moderate to heavy loads, with the boost dropping off at higher rpms. This seems to be "normal" for the wastegated 6.5L.

However, twice now the boost has suddenly dropped to zero for no apparent reason. The first time it happened, I was pulling away from a stop and I glanced over at the guage and it showed no boost whatsoever. While I was watching it, it suddenly jumped up to 6 psi and I could feel the power come back on.

Last night I was on the interstate doing 75 mph, when the boost dropped off again to zero. The needle was on the peg. It stayed that way the entire time until I pulled off the interstate and came to a stop. Pulling away from the stop the boost came back up again. Once I got back on the interstate and approached 75 mph, the boost dropped back off to zero. Turning off anto another smaller highway, the boost came back up to 6 psi and stayed there the rest of the night while I did 60 mph.

Since the boost seems normal most of the time, I would assume the vacuum pump and wastegate solenoid are fine. What do you think is happening? Is the PCM not liking certain conditions and allowing the wastegate to open and drop the boost?

Casey

[JohnC]

Most likely the wastegate solenoid is failing.

Next time it happens on the highway, back off the throttle and come back on it. the boost wil probably return. Watch the boost level and see if it creeps up then drops out, indicative of an inability of the PCM to control boost followed by the PCM shutting the solenoid down. Often this behavior can be correlated with high underhood temps.

The Check Engine light should come on when it happens and the PCM should set DTC 78. If it doesn't, look at the vacuum pump instead. Plumb a gauge into the line from the pump to the solenoid. If vacuum drops off when the problem occurs, find out why...

[DennisG01]

Well, never count out that wastegate solenoid going bad. There's a check valve in there that starts to corrode. Try spraying some WD-40 into the vac line at the wastegate actuator while the engine is running. Do this a few times over the course of a few days. This is also good PM to do every so often. Are you getting any codes?

[Robyn]

Do a few checks

Using a good vacuum gauge, check the vacuum at the vacuum pump. This should be 26" HG and steady. If its lower than 20 and or the needle wiggles toss the pump and get a fresh one.

Check all the vacuum lines from the pump to the solenoid as well as to the actuator.

If the pump is good as well as the lines then the solenoid is supect.

When the system is working right there should be a nice steady vacuum at the actuator.

The solenoids are notorious for taking a hike so unless you find other items bad I would invest in a fresh one.

If the boost drops off and you are on it you should see a code set (SES)

I have seen this before and when the boost goes away the black smoke rolls and within a few seconds or so a code will usually set.

"Wastegate solenoid fault"

This can be due to lines, pump, solenoid or ???

My recommendation is a chip that will allow a turbo master and be done with all that stuff that sucks.

I have a Turbo master on my Dahooooley and the thing is great.

A chip that will allow boost without tripping a code is sweet. You can drive the thing on the gauges and be well off.
Set the TM to peak at 7-9 PSI and your good to go.

If you have a performance exhaust and the chip then you can crank the boost up some more 12 Max without a cooler

[arveetek]

Thanks ya'll!

No SES. It's never been on since I've owned this truck. Makes me wonder if the bulb is out!!

Casey

[DennisG01]

Thanks ya'll!

No SES. It's never been on since I've owned this truck. Makes me wonder if the bulb is out!!

Casey


I wonder if that would set a code.........:rolleyes:

[KallyI]

After installing my turbo boost guage on the Tahoe, I've been able to monitor what's really going on under the hood. Most of the time, the boost seems normal, with a max of 7 psi under moderate to heavy loads, with the boost dropping off at higher rpms. This seems to be "normal" for the wastegated 6.5L.

However, twice now the boost has suddenly dropped to zero for no apparent reason. The first time it happened, I was pulling away from a stop and I glanced over at the guage and it showed no boost whatsoever. While I was watching it, it suddenly jumped up to 6 psi and I could feel the power come back on.

Last night I was on the interstate doing 75 mph, when the boost dropped off again to zero. The needle was on the peg. It stayed that way the entire time until I pulled off the interstate and came to a stop. Pulling away from the stop the boost came back up again. Once I got back on the interstate and approached 75 mph, the boost dropped back off to zero. Turning off anto another smaller highway, the boost came back up to 6 psi and stayed there the rest of the night while I did 60 mph.

Since the boost seems normal most of the time, I would assume the vacuum pump and wastegate solenoid are fine. What do you think is happening? Is the PCM not liking certain conditions and allowing the wastegate to open and drop the boost?

Casey


My truck started doing the same thing a week or so ago. I checked the vacuum, and it was very low. Within a day, the vacuum pump started being very noisy, so I assume I must replace it now. Maybe it explains my mysterious "fishbites"?

[JohnC]

No SES. It's never been on since I've owned this truck. Makes me wonder if the bulb is out!!

Does it come on when you turn the key on?

If a tree falls in the woods and no one is around to hear it, does it make a sound?

(Alternatively, if a man says something where there is no woman around to hear, is he still wrong?)

Check for codes. I'm betting it has DTC 78 stored, and maybe DTC 46 (MIL malfunction)

[arveetek]

Does it come on when you turn the key on?



That's a good question! I've never paid attention. I'll have to look when I leave work this evening.

Casey

[arveetek]

Does it come on when you turn the key on?



My SES light does not come on when the key is turned on. I suspect it's been tampered with. When I bought the truck, I discovered odometer tampering through CARFAX. Luckily, I was able to use that to leverage a better price! I wouldn't be surprised that whoever rolled back the odometer also disabled the SES light.

Casey

[JohnC]

Get a scanner and check the codes. As far as the light goes, chances are the bulb is missing.

[arveetek]

Yesterday afternoon it got a bit worse. As soon as I hit 70mph, the boost dropped off to zero, just like a switch had been thrown. After a while, I had trouble maintaining boost when doing only 65mph. If I gave it much throttle, the boost would drop out. Most of the time, just letting off the throttle and getting back into it restored the boost, unless I was over 70mph.

Last night I temporarily hooked the wastegate pod directly to the vacuum pump. Now the boost holds steady! I saw a peak boost of 13 psi, but one time under full throttle, high rpm, and high boost, the computer must not of liked those particular readings and de-fueled the engine. I had to shut the engine off and turned it back on and then it was normal again. Still no SES light. I can't imagine how many codes are stored in there now!

Looks like a Turbomaster is in my future!!

Boy, I can sure see how someone could easily damage a 6.5L when towing large loads. Without gauges, a person might not realize the wastegate has opened, dumping the boost, and then allowing the EGT's to skyrocket, and then the throttle would be mashed even more to try and make up for the lost power. Sheesh, what a scenario.

Casey