Engine stutters / stall at low speed close to idle, letting off throttle it pops

[tkruger]

1987 Monte Carlo 4.3 TBI.

This only appears to be occurring when the engine has fully warmed up. I have two separate issues that I suspect may be connected.

1. While at low, creeping speed, often with the brakes on the engine will idle real low and stall. It has no issue restarting. Notice this mostly entering the garage or parking the car.

2. This does not occur when the car is in gear. I rev the engine above 3/4 throttle and let off quickly. Multiple pops, similar to backfires will be heard.

Any ideas where to start to look for this? I had thought fuel or timing but other than these two instances it drives fine. The plugs all look good, and the wires and plugs have under 5k on them.

 

[olds307 and 403]

Fuel pressure and timing are good starts. The TPS, IAC and Map sensors are easy to check as well. No codes?

 

[Sweet_Johnny]

Engines should run smoother at operating temp, so only having issues then indicates that the problem is exacerbated by heat. That's typically going to be fuel or ignition related. I had an ignition module go out on me, and the symptoms were similar. They act up/ get worse once warm and can cause an erratic idle and power loss, especially up high. I also noticed problems with smooth rpm climbs while easing away from stoplights and trying to cruise at 30 mph. I adjusted the carb, the timing, changed the advance weights, looked for vacuum leaks, bad plugs, wires, etc. all to no avail. They're about $30 and if you're going to check it, just replace it.

 

[olds307 and 403]

Make sure the cap and rotor are also new. The small cap on TBI trucks had plenty of corrosion issues.

 

[78Delta88]

TPS and IAC is good start. TPS wears out and ECU loses where it is momentarily, so there are tests you can do with VOM and full sweep, closed to WOT.

IAC gets crudded up real easy, pull it out and spay with carb cleaner and put it back in. Crown Vics were terrible with this.

Air pump diverter will cause popping when going from part throttle to closed on a warm engine. Check to see if vac line didn't fall off or break.

Also check O2 sensor or depending on milage, just replace it.

This is the easy stuff...

Also pull the top of the cleaner and notice the spray from the nozzles. TBI typically runs rich at idle. Check the spray pattern. When going from off idle to curb idle you should see good spray, good circular pattern. Oreilly Auto still has the fuel pump tester to rent and It has the fitting you need for TBI, 12mm fine thread. Should be 9psi at idle.

Then also check valves... Run up to 1000 to 1200 and hold, park or neutral, wheels chocked. Place small piece of paper on rear exhaust exit. If the paper sucks into pipe you have exhaust valve issue. This will cause popping but won't be noticed while driving.

 

[tkruger]

Thanks for all of the suggestions. There are no codes, no cel. I am going to try replacing the coil first. Looking at it I saw it was riveted to the mounting brackets so I suspect it may be the original one. At $30 it is worth my time to just replace it. While I am at it I am going to put in a PVC valve, breather, air filter and anything that is general maintenance in the area. Will have a look at the vacuum line that are visible with the air cleaner removed also.

 

[Sweet_Johnny]

While the coil may need looking into, they typically make starting the engine more difficult when bad. They also commonly trip the CEL. The ignition module is another part entirely. I see they've increased in price since the last time I looked, $30 is on the low side.

If the coil isn't bad then you gain nothing by replacing it- the stock ones are just fine even in performance applications. Failing modules are common enough that many people (myself included) keep a spare on hand. Or you can switch to a digital ignition and forget the module.

Just curious- What exhaust system is on this car, front to back?

 

[tkruger]

If the coil isn't bad then you gain nothing by replacing it- the stock ones are just fine even in performance applications. Failing modules are common enough that many people (myself included) keep a spare on hand. Or you can switch to a digital ignition and forget the module.

Just curious- What exhaust system is on this car, front to back?

Except for the tires and the AC being converted to use modern refrigerant everything is stock. The cat has been replaced in the past and everything is flowing well. The pipe from the cat to the muffler is the oldest part and needs to be replaced soon, used JB-Weld to patch a hole and get through the season.

Auto Zone had a coil for $30 and I figured I would try it. If it is not the issue I will just have a spare on hand. Will know more when I have time to change it this week.

 

[Clone TIE Pilot]

I am only familar with CCC systems but if TBI is similar to CCC, then TBI should run richer during open loop when the engine is cold. All cold engines require a richer mixture to start. Once it warms up and enters closed loop it should run leaner, this is when vacuum leaks show their ugly heads and lean out fuel mixtures, especially at idle. Not just vacuum lines can leak vacuum. This is where a decent OBD1 scanner that displays live sensor readouts comes in handy.

 

[78Delta88]

TBI is similar as far as ECM control.

The injectors run hot and ECM grounds them to make them pulse.

ECM get it's que from Distributor module, as pickup coil in unison with module acts as the cam position sensor.

Coil fires on the ground side collapses the primary windings and reverse current builds in the secondary and jumps the plug gap when rotor is lined up to the wire contact.

Injector is similar..., pulsed by ECM in relation to RPM signal from module. In simple terms..., injectors stay in sync with what the coil is doing.