88 TBI? problem

i'll try and keep this short. have an 88 with a 350 w/TBI. used to run fine and then without warning, will no longer idle. i can get it running and actually drive it down the road but as soon as i stop at a light or put it back in park, if i dont keep my foot on the gas, it cuts right off. over carbing bad, dumping fuel into the throttle body. service engine light doesnt come on but the map sensor code is stored and wont clear. i've already changed the map sensor, pcv, fuel filter, oxygen sensor, iac valve, general tune-up. tps checks good but hasn't been replaced. one shop, without checking anything, said they thought it was a head gasket?, but the truck has plenty of power, runs fine at speed, no water in oil, etc. any ideas?

Sounds like the EGR valve or EGR solenoid is stuck open.

how can i check that short of replacement? can i just disconnect the egr and see if the problem goes away? plug all vacuum hoses going to egr?

OK, the EGR valve is located on the passenger side of the TBI, it looks like a mini UFO (flying saucer). Take off the air cleaner assembly and with the engine cold so you don't fry your fingers, feel around for an opening, stick your index finger inside and pinch against your thumb. You should feel the spring loaded diaphragm open with moderate pressure, if you don't (in your case) it probably cause it's stuck open. On my truck you have to pinch very hard to open. Maybe the guy who wrote my manual is a guerrilla or I'm weak.

The EGR solenoid valve is upstream from the EGR valve you need a vacuum pump to test it's operation. To test you pull the vacuum tube off thats between the valve and the manifold and put a vacuum pump onto the solenoid valve. If the valve holds a vacuum it passes the test.

My manual says some vehicles are equipped with an electronic Vacuum Regulator (VRV), these have a filter that should be cleaned with soapy water. I assume these replaced the EGR solenoid sometime after the 88's.

I hope some guys can jump in on this thread with other things to check besides the EGR. I'm only going from my repair manual says is the most likely cause of idle stalling after the IAC and given it's running well at higher RPM's

Needs a new manifold-TBI gasket...........

my 91 had the same prob. and it was a bad intake gasket

Simple stuff first. I would see if your ignition module is original( located under distributor cap). Have it checked for free at most parts stores. Plugs, cap, rotor, wires OK? Ohm out your coil and make sure it's within spec. Do the same to your pick up coil under your rotor. The coil, module, and pick up coil can cause hard running, hard start, no start conditions. And they will not set a check engine light as they are not monitored by the computer. Egr is another good suggestion. But check the basics first. Make sure you have solid grounds everywhere, engine-to-frame, engine-to-battery, battery-to-frame, etc. Good Luck -Chris

Oops...almost forgot, check your MAP sensor wires with an ohmeter. Should have a 5v reference on one wire, ground on another, and the last goes to the computer, you can't test that one per se. And make sure your fuel filter is clean, that your getting 5-13 psi fuel pressure, and that you don't have ANY vacumn leaks.

my intake leak was a sort of vacuum leak i guess you would say. the intake gasket broke where the exhaust passage is in the intake. i found out by accidently spilling a glass of ice water on the intake while i was checking vacuum lines etc.

i'll try and keep this short. have an 88 with a 350 w/TBI. used to run fine and then without warning, will no longer idle. i can get it running and actually drive it down the road but as soon as i stop at a light or put it back in park, if i dont keep my foot on the gas, it cuts right off. over carbing bad, dumping fuel into the throttle body. service engine light doesnt come on but the map sensor code is stored and wont clear. i've already changed the map sensor, pcv, fuel filter, oxygen sensor, iac valve, general tune-up. tps checks good but hasn't been replaced. one shop, without checking anything, said they thought it was a head gasket?, but the truck has plenty of power, runs fine at speed, no water in oil, etc. any ideas?

Over carbing/ dumping fuel. Check the fuel pressure regulator diaphram in the t/b it probably has a hole in it. You can get them at Auto Zone cheap. :wink:

I would check for an intake leak by spraying carb cleaner around the TBI base and where the intake manifold meets the cylinder heads, if you have a leak the idle will raise while spraying near the leak.