Main IndexAuto Repair Home Search Posts SEARCH
Who's Online WHO'S
Log in LOG

Search Auto Parts

Vacuum leak?

  Email This Post


May 9, 2015, 8:16 AM

Post #1 of 2 (714 views)
Vacuum leak? Sign In

So i have a 1989 GMC 1500 with the 4.3l, recently ive been having some idle problems. especially when under load. Ive tried to remove codes from the OBD, only to get a code 12, (fine). so after staring under the hood for a few hours i replaced the following items; EGR, PCV Valve, IAC, MAP sensor, O2 sensor, and probably a few more things that i just cant remember. So after looking for vacuum leaks i found a small one and replaced the hose. but still has a pretty wicked idle. I disconnected the MAP sensor plug and it ran pretty clean, maybe a little high idle.
Also, for some reason the engine doesn't want to start if i have the hose from the PCV to the throttle body connected? but as soon as i disconnect that it fires right up?
I'm pretty confused with this, anybody have similar issues?

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 9, 2015, 9:23 AM

Post #2 of 2 (707 views)
Re: Vacuum leak? Sign In

Slow down on just tossing parts at it as it's probably confusing the real issue.

The only good reason an UNPLUGGED PCV hose to throttle body base can help IMO (no help from codes no surprise) is it's running way too rich and that provides air usually too fast to run if off.

If so this should have soaked plugs with fuel now and possibly oil may read too full and smell of fuel - not good.

So. Can you get this running and take an actual manifold vacuum reading on a "T" so line still feeds wherever it's going? If normal it should read 18+ Hg or so with no load with engine running. It would be of some info and try again with hose to PCV plugged in if it will stay running.

Look at plugs on that list of course. Another item that can send signal for lots of extra fuel is the coolant temperature switch (CTS and other names) meant only to tell engine how much fuel to deliver while cold and less as it warms up. Default if disconnected either on purpose or not making a connection engine will deliver fuel as if it's ~40F below zero - too much of course. You can ohm test the sensor noting close engine temp and I'll look up if it's close. CTS is on or near thermostat housing usually with black and yellow wires if original plug and wire. Don't confuse that with sender for temp gauge it's different.

That's a couple things you could do. No codes isn't helping. Even this old that should have codes and possible that's the problem? Unplugging stuff with key on or engine running would set codes for several things already. That's not such a good test if an item is good or not BTW,


  Email This Post

Feed Button

Search for (options) Privacy Sitemap