Mysterious AC and Loss of Boost

Emmo

Emmo

Member
Hi all,

I've got a odd question that I've been trying to answer for for quite a while on my basically stock Sy. Every now and then when driving around for a short or long period without putting any stress on the engine (hitting the throttle to boost) my cold AC mysteriously cuts off and blows plain air. At the same time, the engine along with the fuel delivery wont allow me to raise any boost when I push down on the throttle. Basically the fuel will cut above 0 psi and the truck will limp along. I'm able to keep up with traffic but not accelerate at any quick pace. Brakes work fine too.

This happens maybe once every 2 weeks at a wide variance of outdoor temperatures for the past 4 years. Truck is driven often. If I then turn off the truck and let it sit for 5 min before restart it runs like normal, AC and boost. And if I continue to drive carefully without boosting or accelerating after about 10min the truck regains AC and runs normal again with boost.

In chasing down this issue and with general shade tree mechanic upkeep Ive:
Replaced all vacuums lines and fittings
Fixed AC hissing in dash panel
New stock boost solenoid
New wires, plugs, rotor, and cap
New O2 sensor

Don't really know what else it could be.... maybe something with the computer or other sensor that's getting old?

Thanks for any thoughts :) Pete
 
Don W.

Don W.

Stab it and steer it
Re: Mysterious AC and Loss of Boost

My first thought is that those two wiring looms are close to each other (come out the same bundle if I remember). Check for abrasion, bad/corroded connections.
 
WyoSyclone

WyoSyclone

Active member
Re: Mysterious AC and Loss of Boost

First thing I'd do is replace all the check valves on the vac lines.
 
gkrcr882

gkrcr882

SyTyless......for now!
Re: Mysterious AC and Loss of Boost

With the AC, does the compressor keep cycling when it blows warm or does it shut off? If it keeps cycling, it's a blend door issue. If it cuts out, it may be a wiring issue or a sensor out of adjustment or just gone bad. If I remember correctly, there should be a low side pressure switch and a high side pressure switch on our FOT system. If the low side switch senses the pressure getting too low, it will shut the compressor clutch off to raise evaporator pressure to prevent freezing. On the high side, if too much head pressure is detected it will also shut off the clutch to prevent damage.

For boost, are you going off the stock boost gauge or an aftermarket one? Has the truck been worked on previous to you owning it (I.E. replacing the MAP sensor with an incorrect one).

Since the problem goes away when the truck cools down, that throws up a red flag that it's heat related. If you have a sensor with bad contacts, once it gets heated up the contacts will open farther and create high resistance or an open in the circuit. Hopefully it turns out to be something simple. Good luck!
 
Emmo

Emmo

Member
Re: Mysterious AC and Loss of Boost

Thanks guys, good notes...

Yep, I'm needing to get a data master setup in the near future. I haven't replaced the old stock MAP sensor yet. Might have something there, which I think scanner will confirm.

The AC clutch basically stops cycling and the blend door works fine - after my heater core replacement/fiasco 3 years ago,

Other note, stock boost gauge and engine temp on dash I replaced 4 years ago with VDO gauges so I'm pretty sure engine temp is correct at just under 180 degrees with no overheating. Stock boost gauge is capped off. But if I remember right there might be a "2nd" engine temperature sensor that sends back to the computer that may be shot?

I blew through the 2 vacuum check valves by mouth recently 3 weeks ago during line replacements and both seemed to seal good so still stock with new hoses. Issue as such happens before and after recent line replacements.

Additional note to replies, the boost does drop completely to 0 zero psi per the gauge, can't get to 8psi. I can slowly accelerate but fuel seems to cut out if I try to push the throttle too much. Final note, I did decide about 3 years ago to run 3" straight pipe after the catalytic converter with no mufflers and ran into over boosting issues when it only got cold outside. That's been resolved by adjustments to the waste gate but this AC/boost seems random to driving conditions.

I'll go the way of the scanning tool - datamaster (both as something I've been wanting to get and help with this issue)

Thanks again for the tips, Pete
 
Emmo

Emmo

Member
Re: Mysterious AC and Loss of Boost

Hi all,

Quick update as of this January. Been driving quite a bit around the Southeast for work with knock on wood no boost issues so far since I posted this thread. It hasn't acted up so I haven't tried Dave P's trial yet of disconnecting the boost solenoid connector. So will post back next time it starts up and I cautiously try it. Weird....

Still need to get a datamaster setup for myself along the way...

Thanks again for tips, as it's bound to do it again, Pete
 
You must log in or register to reply here.
Top