'89 350 tbi 5 speed stalling and surging idle

[bowtie5]

Hello everyone and thanks in advance for your help.

I have been experiencing a surging idle with stalling at stops or as I slow down. I have a 5 speed so I dump the clutch and the idle drops very low and rough.

Had this problem when I bought the truck. I did plugs, wires, cap and rotor. Had bad fuel pump and I replaced filter with it. Took it to a shop to diagnose the idle and it had a map sensor code. Shop replaced map sensor and egr, as well as a new water temp sensor and 205 degree thermostat. Didn't stall for quite a while, but the idle was still a little low at stops.

There are currently no engine codes showing. Fuel consumption seems to be quite terrible lately too.

Hope I have given enough info. Thanks again.

[54 jimmy]

check you throttle body gasket for a leak. an easy eay is to spray 2+2 carb cleaner at the base of the throttle bodyif it stalls out that means the gasket is bad. ot the pcv valve opening in the throttle body could be plugged that would cause back pressure in the motor an couse it to run crappy. these are the things i ran into on my s10. they are basically the same. hope this helps.could also be a vacume leak.

[ChevyTech]

A common cause of an idle that hunts up and down on these trucks is a lean fuel mixture, usually from low fuel pressure. The fuel pressure must be tested with the engine running. An IAC sticking and a worn distributor can also cause idle issues.

There are multiple failures that can causes the TBI trucks to have low fuel pressure other then the fuel pump.


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.


When the engine does start, 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.

Does the engine start well without the gas pedal being pushed down at all?

[bowtie5]

Thanks 54jimmy for the advice. I sprayed carb cleaner aroung all vacuum lines and tb base with no change. PCV is new and working properly.

I do not have A/C to test that part.

When I start in the morning it doesn't seem to run any higher than when it is warm. Other than cranking a few seconds longer than I like, it starts fine. I never press the pedal when I start the truck. I assumed that it didn't do what a carb does. Other than the constant surging it runs fine. It doesn't surge from normal rpm to a higher rpm, it goes from a normal rpm to a low (rough) rpm.

Paying as much attention to the problem as I can lately I have noticed that the roughest idle and stalling happens after a hot restart. For example going to a store. I will come back out after 10 or 15 minutes and start the truck. It will idle very rough and stall each time I slow for stops signs or lights for a good 2 miles or so. After a few miles it goes back to just the surging.

Is there a way for me to test the IAC or distributor? Like I wrote before the shop hooked up the computer and showed everything running normally.

Thanks again for the advice.

[54 jimmy]

check your ign module under the distributor cap. mine would run fine untill it got hot the it would just stall/die and wouldn't start again untill the mod was cooled down. mine had a hole burned right through it. your local auto parts store can test it to tell you if it is bab. last time i replaced one they were like 25 bucks.

[ChevyTech]

Check the coolant temp sensor on the intake manifold. The computer uses the coolant temp to determine how much fuel to inject and how fast the idle speed should be.

There is test information posted at the thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=379142

[AverageJoe]

I had these problems and solved them by buying a new intake and a CARB

[ChevyTech]

Thank you for the kind words K5hart.

When I start in the morning it doesn't seem to run any higher than when it is warm.

Something is wrong.
You replace the IAC.
The IAC should always speed the engine up at least some, when starting the engine, even if it is warm.
The passages could be plugged. The ECM could be bad. The wiring between the ECM and IAC could be bad.

Had bad fuel pump and I replaced filter with it.

Just because you replaced the pump does not mean the pressure is good. There are multiple failures that can causes the TBI trucks to have low fuel pressure other then the fuel pump. It could have a fuel pressure regulator problem.

Just because the pump is new does not mean it is good.

I suggest only using OEM (Original equipment manufacturer) fuel pumps. I have seen so many pumps from other manufactures fail in the warranty period or just after the warranty is up that I suggest only using an OEM pump.

Took it to a shop to diagnose the idle and it had a map sensor code. Shop replaced map sensor and egr, as well…

The exhaust could be restricted. Does it sound unusually quite? This would explain the poor fuel mileage too.

If the IAC seemed to be working ok, I would say you should test the fuel pressure with the engine running.

[bowtie5]

Thanks k5hart. I have put a new egr on. I push up on the diaphram while it is running and it stalls. That is the quick test if it is operating I have heard. I may be overstating the symptoms a bit. This is by no means a reason to park it. It is still my daily driver and I love the truck. Just want it as right as I can get it. You are right, chevytech is the walking, breathing chevy encyclopedia.

Believe me averagejoe, I have thought about grabbing the edelbrock carb and intake combo in the garage, but all of the horror stories I have heard stopped me quickly. Thanks

Hi Chevytech. I am very appreciative of all of your expert advise. I don't want to make any mistakes in my descriptions that will mess up your train of thought. So let me go back a bit.

As far as the cold idle speed... I don't have a tach and the previous owner thought flowmasters that exit in front of the rear wheels was a neat idea... so There may be at least some difference in idle speed at startup. I have a tach that i am going to zip tie to the cowl to verify this for you.

When I replaced the IAC I didn't take the time to check and clean all of the passages. I will do that tomorrow when I hook up the tach.

No exhaust restrictions that I can feel. If you rev it up a little it will blow your hand off the tailpipe (sidepipe).

I guess I need an inline fuel pressure gauge for that test? My buddy at the shop could handle that for me I suppose. What fuel pressure should I be looking for?

Once again I don't want to over exaggerate the symptoms. It does surge and occasionally stall on restarts. Fuel consumption is about 10 to 12 mpg. But it is a running ,driving truck. Just enough wrong that I think it needs addressing. I did forget about one thing I observed.... It sometimes deisels when I shut it off after a long drive. By long I mean not a short 5 minute drive. Anything over 20 minutes or so, Not terribly, but for a second or two it does run on. I checked the timing was at zero with the connector unplugged, after the map, egr, etc was done at the shop. I will recheck that tomorrow with my other tests.

Thanks very much everyone

[ChevyTech]

What fuel pressure should I be looking for?

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

TBI engines should not “diesel” (run-on) when shut off because there should be no fuel. Make sure the injectors do not drip fuel when it is shut off.

When you replace, remove and cleaned an IAC, or unplugged the IAC with the key on, the ECM may need to relearn the IAC position. This will not happen without driving the truck and meeting specific conditions or taking other steps the cause it to relearn the idle.