OK, we all know that 'overboost' is typically a leaky vacuum/boost line. Not always but I'm going to guess its a leak rather than a mecahnical or electronic issue 80% of the time. Now that number was PFA (Pulled from ass).
I have an 85 Shelby Charger that was recently gone through and it initially had a pretty bad overboost problem. It did not take much effort of the right foot to cause an overboost and Check Engine Light (CEL). I went immediately for the vacuum harness as the root cause. A pressure source of 15 lbs helped me find the leak in like 5 minutes. I used my $59 3 gallon Harbor Freight air compressor to push 15-20 lbs of PRESSURE into the vacuum harness and the leak in the wastegate actuator was easy to find. It was at the factory connector where all the vacuum lines from the engine are plugged into the lines from the solenoids. Its the factory setup, its made of rubber, its round, it has a number of fittings on it and it makes it easy to plug and unplug the vacuum harness for working on the engine. It LOOKED like it had never been messed with. WRONG!
After I fixed the leaky wastegate line, the overboost was now corrected, but it knocked/detonated with even a few pounds of boost. Rechecking the timing, and the other common setup items didn't uncover any new problems. No fault codes.
On a whim, I decided to first (also) confirm that the vacuum lines were all properly routed and going to the correct solenoids. In brief, they were not!
Apparently, at some point in the cars past, two or more of the lines must have popped out of the rubber connector(s) and the PO put them back on the wrong holes! I forget exactly but the MAP
solenoids were not properly routed and going to the wrong solenoids. It might have been the purge circuit as well, Im not sure but the first two I checked were just flat out wrong. I thought maybe I was not following the right line from one end to the other at first because it all looked so factory fresh and unmolested.
WRONG WRONG WRONG.
I wound up actually removing the factory connectors where the two looms come together over by the FPR
, and I spliced the lines together with an appropriate size black silicone hose and zip ties. Even with the connector fully zip tied together it still leaked under simulated boost. I could have used RTV to seal it all up but then it would not have been very servicable being all glued together. The emissions diagram from the FSM
was used to ensure proper routing and connections.
I'm not saying....I'm just saying. Don't assume they are right and dont leak because they look great. Test them w/pressure and check the routing! I blew a brand new engine a few weeks back that I spent almost 2 years building (in my spare time) because of detonation under boost (only 15 lbs!) and it only took about 1 second OR LESS of WOT
to do it. I broke a ring or a piston or both, dont yet know because the engine is still in the car...but you can bet that after the repair of the big problem, I'll be dbl checking the routing and doing a great pressure test on that cars vacuum harness as well.
We can't say it enough...detonation/knocking under boost KILLS!