The vacuum lines need to be replaced. This is generally a result of low engine vacuum.

 

I've replaced the vacuum lines just last season. I guess they could have gone bad again. Checked the lines with carb cleaner just last week and no reaction though.

I havent checked all the vacuum lines, just the major ones, so maybe thats the problem. I will do a recheck and check all the vac lines just to be on the safe side, thanks for the tip!

What I don't understand is how a vacuum leak could depend on the speed of the vehicle, because nothing is wrong with it while standing still. Shouldn't a vac leak cause it to surge while pumping the breaks in park or neutral gear?

As I've said earlier, there's nothing wrong while the car is standing still.

Thanks for the tip though, I will check it out tomorrow!

 

Not while the car isn't moving. My 87, when I bought it, ran just fine in my driveway. Anyplace other than that, before I replaced the lines, it stalled pretty much every time I hit the brakes (had to powerbrake). The ECM uses a different set of calibration tables when moving than it does when not. Could also have dry-rotted injector o-rings or valve cover gaskets, or badly sealing PCV system components in the valve covers.

Pull the vacuum line off of the MAP sensor by the air conditioning duct box and look for anything that could come out. If anything comes out, the MAP is shot. Test the output voltage from the MAP between the green and black wires with the engine running.

 

I have been having the same problems with my 3.1L. I have only owned the car about 3 weeks and I was starting to go insane looking for ideas on how to fix it.I have cleaned the IAC,changed plugs&wires,fixed vaccums leaks,replaced PCV.I am going to get into the vaccum lines again and hopefully replace everything.

My car has been dying when I decelerate coming off the highway.Doesnt die any other time,but I have noticed some rough idle when braking at lights.And this only happens at operating temperature.Wil let you know if I find anything that might help.Gonna work on it and research more online.Cheers

 

Ok, that would explain it. How do I check the PCV components though? I've checked the PCV valve already (been down this road before). It rattled when
I shook it, thats means it's OK, right?

I've also made a (almost) complete rebuild of the engine about 3 years ago. So all the gaskets and o-rings are pretty much new. So there should be no problem there, but it can't hurt to spray some carb cleaner on the injectors I guess...

Will check the MAP sensor later this week.

Thanks!

 

Check to make sure that the tube that attaches to the PCV valve is sealed against the nipple that actually attaches to the valve AND to the nipple that attaches to the lower intake. Check both nipples for dry-rotting and cracks. Check that the PCV valve isn't so loose in the valve cover grommet that it has the ability to move around (should be tight, but loose enough to get out). Check that the tube in the driver's side valve cover is undamaged, that the elbow in the valve cover is undamaged and seals against the cover, and that where the tube runs into the intake bellows isn't damaged as well.

If you have oil leaks near the spark plugs, chances are that the valve cover gaskets need to be replaced and they're leaking vacuum. Or they're just loose. Also, check that the T in the back of the upper plenum is screwed into the plenum tight, and that the check valve in the brake booster (where the big hose connects) is sealed by spraying some carb cleaner or otherwise on it. Check the nipples on the end of the hard lines to make sure they aren't damaged as well.

 

I've checked pretty much everything now. No oild around the sparkpluggs, pcv seems good. Sprayed so much carb cleaner that you get high just by opening the hood, no reaction. All the vac line connections are good also.

Unplugged the vac line to the MAP, nothing "came out". Couln't find an easy way of meassuring the voltage without piercing the wires tho, so I'll leave that for now.

Pulled out the IAC, easy clean and put it in again, no change. The IAC is new as of last summer.

The car died today while breaking on the driveway, haven't done that before. So I tried disconnecting the brake booster and plugged it up. Doesn't stop while breaking on standstill anymore (no **** sherlock) but the other symptoms are still there. Still stalling while coasting.

The idle goes all over the place while breaking on standstill, does it mean that the brake booster is bad? Or could it be related to the other symptoms (the brake booster stealing vacuum and disturbing the "system")?.

I've got some noise from the smog pump/air pump, could it be related to that? And the gas pump is making quite a lot of noise also.

I was thinking about the purge valve on the gas evaporation canister, could it cause these symptoms if not working correctly?

Just throwing my ideas out there, anybody got a clue? =)

 

Unplug your EGR valve.I removed mine and now it idles fine and no longer stalls.It didnt fix every problem I have,but most of them.My car now runs reliable.

 

You can disconnect the large hose running to the canister from the throttle body and plug it to see if the valve is leaking. The hose should run into the 2-port thing on the top of the canister. And yes, braking while standing still shouldn't change the idle much, except maybe for the computer to increase the idle speed a bit to compensate. If the idle speed drops, the booster is indicating that the engine is running on low vacuum.

How old are the intake gaskets? Have they been replaced at any point while you've owned the car?

 

Like I said earlier the engine has had a complete overhaul only 3 years ago, and has only run maybe 10k since. So the intake gasket is in good shape (sprayed carb cleener over both lower and upper intake, and no reaction).

Are you still sure it's a vacuum issue Maverick? I'm inclined to agree but by now I'm starting to doubt myself. Aren't there anything else than vacuum leaks that could cause these symptoms? Or is it just (as I believe) that vaccum leaks is the most probable issue?

I'm going to plug the canister tomorrow or the day after. Gonna remove the EGR and check for leaks also.

 

If the engine has only had an average of a little over 3k per year, that means it does a lot of sitting, right? That leaves plenty of time for the valve cover gaskets and other parts to dry out and cause problems.

However, the only way to really find out what is wrong is to get both a vacuum and a fuel pressure gauge. Vacuum should be over 15" and fuel pressure should be between 37 and 47 PSI.

Also do a resistance test of the fuel injectors. Should be no less than 11.8 ohms per injector. Any lower, and the injector needs to be replaced (as according to GM).