1993 suburban very low idle in gear

[tedfrk]

ok so here i go,it has a 5.7 liter,auto trans,i brought it to the shop because everytime it is put in gear or while your sitting at a stop light the idle drops down to around 100 rpm's on the factory tac and when you go to take off it bogs and almost feels like it is going to die but then you get going and it seems ok until you stop again.i had a code 32 and the egr was replaced.no code 32 any more but still the same thing the guy at the shop had it for 2 whole days a could not find anything wrong.any help at all would be greatly appreciated .

[N2TRUX]

Two things come to mind. One is low fuel pressure and the other is weak spark. How long since you replaced the fuel filter? How many miles does it have on it? It could be a weak fuel pump, but I would consider the filter first.

Regarding spark, I had a 95 Chevy truck that the coil was going bad. It would idle down and bog at low speeds. Anything above low speed and it seemed fine. Once I replaced the coil it was amazing how much better it ran.

[ChevyTech]

Question:
When the engine is started, does the engine have a fast idle that slows down as it runs? The IAC (idle air control) is what gives the fast idle speed when started cold.

The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage.

The IAC (and computer) control the idle speed.
The IAC (and computer) is what gives the engine a “fast idle” speed when started cold.
The IAC also supplies more air to compensate for the extra load when the A/C is on. If you have A/C and turn it on while listening carefully you should be able to here the IAC controlling the idle as the A/C clutch engages and disengages.

IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

[tedfrk]

the idle is only fast when it is first started up and slow down when warmed up.the fuel pump was just changed as well but had this problem before.i will check tomorrow to see if the a/c effects it .when i push on the brakesit is fine but when you put it in any gear it goes down to about 100to 200 rpm on the dash tach.

[ChevyTech]

the idle is only fast when it is first started up and slow down when warmed up.the fuel pump was just changed as well but had this problem before.i will check tomorrow to see if the a/c effects it.

Being it will idle faster, when started cold, the IAC is working.

The next thing that should be checked is the distributor shaft bushing condition. Take the distributor cap off and see if the shaft will move from side to side.

On these distributors the housing/upper bushing area gets so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. The parts get damaged and the ECM receives compromised signal for RPM and timing issues.

[tedfrk]

the distributor is fine,i just changed the coil, spark plugs,distributor,rotor,the throttle positioning sensor,and the idle air valve,still no change.tomorrow the fuel filter and wires,then i dont know were to go.if i take off the vacuum line going to the egr valve and cap it,it wont bog down when i accelerate from a dead stop but still idles low.please help i am going crazy.

[ChevyTech]

As N2TRUX said, test the fuel pressure.

TBI systems do not have a fuel pressure test port.

The fuel pressure is tested by using fittings to tee in a pressure gauge. I like to tee in the pressure gauge where the fuel filter is located. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If pinching off the return line makes a low pressure reading surge above the specification pressure, then the regulator is the problem.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.

Here are some web sites showing fuel pressure test equipment. The first tool gets installed where the fuel filter goes. Most people use the tool in the third site next to the TBI unit.


I you already have a pressure gauge like shown in the second web site below, all you need is the tool shown on the first web site. If not, the third would get the pressure tested for the least money.

http://www.sjdiscounttools.com/ta37650.html

http://www.actron.com/product_detail.php?pid=16174

http://www.actron.com/product_detail.php?pid=16175

http://www.cfm-tech.com/catalog/fuel...er_2940807.htm