SES light at 75 mph

[quickv6]

When driving 75 MPH and under load the SES light comes on. The boost is around 8psi. I can drive a 70 under load with boost in the same range and no problems. I do not have a scan tool to check codes but I am sure it is boost related. I also notice that power is reduced until I pull over and cycle the key. Then all is good again. Is the boost to high causing this problem?

[DmaxMaverick]

What year truck, engine (L56, L65)? It would be helpful if you added this info into a signature. Click on "My profile" near the top of the page and add one.

Could be fuel or air restriction. Have you replaced the filters recently?

If you have a '95 or earlier (OBD I), pull the codes with a paper clip. '96 or later, you'll need a scanner. Either way, get the code read.

[GMC Hauler]

if it is a 96 or newer, you can go to most parts stores (Shucks, Autozone) and use their code reader for free.

[quickv6]

1994 6.5 and the code is 78. I have replaced the boost solenoid about 2 months ago. It only does it at 75mph or higher. The truck will pull all day at 70mph and 10 psi no problem. Creep up to 75 and will set the code 78 instantly.

[Barry Nave]

5Yrs ago I had a long block installed. Boost was good,all was fine, I thought.
Hooked my TT and headed north,came to a grade,boost went to 13psi. SES light came on,boost droped and smoke flew. Backed off,light went out and boost came back. :confused:
Found out that when block was R&R the mech.split the rubber boot at the waste gate can.
Small leak. would not hold boost over 10psi. Search TDC 78 and you will fine a indepth trouble shooting guide. I just use a Vac. brake bleader to check turbo lines for leaks and if it will hold a vaccum. Over boost will through a 78 also. At the time I was also using a indash boost controller that was an Feature Articles by JB years ago. Now I'm running JK boost control with out any issue.

[arrowheadracing]

Quick V6,
I just recently had the same problem with the DTC 78. I was going to a buddys shop to pick up some work and hit the throttle and all I got was black smoke and no acceleration like it ususally does. After investigating things I found the boost solenoid was bad. Which when I tested it ( being 10 degrees outside ) I broke the stupid plastic line. What bums me out more is I was going to show off my new 4" exhaust system by pulling a hole shot in his parking lot, but could only tell him to listen to the truck idle so I wouldnt be laughed at when the kid on a skateboard blew my doors off.

I just read your post on the sticking waste gate arm. And was wondering if that took care of the DTC 78 code you where getting ?

[quickv6]

Yes it did fix the code problem. The truck would show about 7-8 psi boost at 75. I guess this was high enough to set a code. The boost never went over 12 psi. Now the boost at 75 is like 5 psi. I am in FL now pulling the trailer over 1000 miles yesterday and no code. There is a big difference towing 7 psi to 10-12. I wish I could have left it alone the way it was and figured out how to get rid of the code.

[MTTwister]

Quick.. - I don't know if you're a candidate for the "Boost Fooler" but you might want to investigate that option.. can be home made cheap and easy, and fools the PCM into believing a lower boost level is "present". Search on that topic.

[arrowheadracing]

Quick,
If your interested in a boost controller I have one on my truck. But found out today I didnt have a solenoid problem, but the vacuum can is bad. Since you cant buy just the can ( atleast thats what I am told ) I am going to make a mechanical controller like the early models had. Just going to do away with the whole computer controlled one. If your interested email me. i liked it , I had the boost set to 10 psi and was happy with it. If I didnt have this problem I wouldnt sell it.

Thanks
Todd

[quickv6]

arrowhead I AM interested. I am new at this how do I find your e-mail address?

Lonnie

[arrowheadracing]

You can reach me at service@customizedcreations.net.

Todd