1972 Jeep Commando 258 Fast Idle and Stalling Problem

[Bob S]

I need help with no idle , starts up fast but go's to a fast idle ( 2500 rpm ?) and then dies. I have been working on this jeep on and off form about ten years and owning the Jeep since 78. I have remove / replaced  and move everything and now I'm putting back . I must have something wrong some where. put on headers and glass packs with duel exhaust on a 258 s6. any thought's.

[Moses Ludel]

Classic Jeep, Bob S...Begin by checking the fuel supply.  Test at the engine/carburetor side of the fuel filter:  1)  confirm the pressure coming from the pump (stock 1972 258 inline six fuel pump is 4-5 PSI at 500 engine rpm), and 2) determine whether there is enough volume of fuel flowing from the filter.  Fuel volume should be one quart in one minute or less at 500 rpm using a brass Tee and fittings from the fuel line into a steel container.  Make certain the fuel filter is not clogged.  If pressure is okay but volume low, consider the fuel filter or sock at the fuel tank pickup.  Testing requires a safe hose, a Tee, a pressure gauge and a metal can kept away from engine heat.  If the problem is not fuel pressure or volume, move to an air leak. 

When you installed the header, the intake manifold gasket may have developed a leak.  If a new gasket, it may not be sealing properly.  A quick check for a manifold leak with the engine running (idling if possible) is to spray a less volatile penetrant like WD-40 (kept away from the exhaust manifold) along the edge of the cylinder head/intake manifold junction.  If engine speed picks up, you have a vacuum leak.  Do the same around the base of the carburetor, you may have a vacuum leak there.

Other sources for a vacuum leak are the brake booster check valve (if so equipped), the brake booster diaphragm or the evaporative emissions vacuum circuit.  If equipped with power brakes, the brake pedal should have vacuum assist after shutting off the engine, at least for a few pumps of the brake pedal.  If brake assist requires that the engine is running, you have a vacuum leak at the diaphragm, a hose or the check valve.

If these issues are not the problem source, you need to rebuild the carburetor and install a new needle, seat and gaskets.  Not clear whether you have a 1- or 2-barrel carburetor, stock or otherwise.  (If an aftermarket Weber or Holley, that's a whole issue in itself.)  If stock YF Carter, rebuilding can make a dramatic difference if you set every specification to factory.  At the magazine, I have a complete step-by-step for rebuilding the later two-barrel BBD with good results:  https://www.4wdmechanix.com/Rebuilding-the-Two-Barrel-BBD-Feedback-Carburetor?r=1.  This is the later feedback carburetor version of the Carter BBD.  Although the BBD is distinctly different than a Carter YF one-barrel, you can glean useful tips from the build.

Moses

[Bob S]

On 5/22/2020 at 9:31 PM, Bob S said:

I need help with no idle , starts up fast but go's to a fast idle ( 2500 rpm ?) and then dies. I have been working on this jeep on and off form about ten years and owning the Jeep since 78. I have remove / replaced  and move everything and now I'm putting back . I must have something wrong some where. put on headers and glass packs with duel exhaust on a 258 s6. any thought's.

Could it be on the kick down on the automatic transmission as thE Jeep will not run more than a couple of seconds.?

[Moses Ludel]

Hi, Bob S...Sounds like you can't get the engine to run long enough for testing any of the things I suggested.  Is that correct?  So the goal is to get the engine to run long enough for further troubleshooting?

If so, try this simple measure:  With the engine not running and key shut off, lift the air cleaner from the carburetor intake throat.  Hold the choke open.  Now open and close the throttle while looking down the carburetor throat or air horn.  You should see a distinct squirting of fuel.  Pump the throttle for several strokes.  If fuel squirts for the first few pumps then stops completely, either the engine is not getting fuel from the gas tank (clogged pickup sock, a bad fuel filter, weak fuel pump, etc.) or the carburetor's float needle is sticking closed.  Either way, the carburetor fuel bowl does not have adequate fuel in it.

Why am I suggesting this test?  Because the engine starts but quickly stalls.  This means there is likely spark but possibly a limited fuel supply in the carburetor bowl...We can go from there.

Moses

[Bob S]

Thanks Moses I will try that at the moment I'm using a gas can . I didn't what to use the new tank just yet. I also put in a new pump and  yf carb. I have a auto trans ,could it be the kick down on the trans also ?

 

[Moses Ludel]

Bob S...You had asked about the kickdown earlier, too.  A 1972 Commando should have a THM400 (GM) transmission with a solenoid kickdown that moves a valve in the valve body to boost pressure and force a shift kickdown.  The solenoid signal originates at the carburetor/throttle.

If the transmission selector is indexing properly so that Park and Neutral are in the right place, you should have no issue when starting the engine in Park or Neutral.  If the transmission linkage is out of adjustment and the engine is actually starting in gear (forward or reverse), it still would not stall the engine unless the torque converter is defective.  You would feel the vehicle attempt to move forward or in reverse, but the converter should allow slip and an engine idle, as it does when you place the selector in gear at a stop.

I would check the shift linkage to rule out any issue, anyway, but starting in gear is unlikely from what you describe.  The kickdown solenoid, if the activation switch is adjusted properly, should only impact performance in the Drive/forward gear modes.

There is a possible problem with the starter solenoid or wiring, however.  You have an Autolite/Motorcraft type starter with a remote (fender mounted) solenoid.  When you turn the key to start/crank, the solenoid switch closes; during cranking there is direct 12-volt current to the coil from the solenoid.  When you release the key, the solenoid no longer applies the 12-volts, but the ignition switch and possibly a resistor wire or ballast then takes over and provides the coil with a reduced, running voltage. 

Check or replace the solenoid, it's very common and inexpensive.  If that does not solve the problem, trace out the ignition feed from the ignition switch through the resistance wire or ballast resistor to the coil.  You likely have an open wire, defective ignition switch or a defective resistance wire/ballast—some reason for no current to the coil after cranking.

For testing purposes only, a simple way to test this possibility is to by-pass the ignition switch with a direct jumper from a hot 12-volt positive source to the coil's positive primary terminal.  Hook up the temporary jumper, and with the transmission shifter in Park, crank the engine.  If it fires and keeps running, you know that you have a defective solenoid, damaged wiring (including a resistance wire or ballast) or a defective ignition switch. 

Also, you should have a breaker point ignition.  If the original ignition has been modified with a Pertronix or other electronic conversion, that module could be defective.  Or the original distributor may have a breaker point issue or worn distributor shaft bushings with a runout issue.  This would cause breaker point dwell to vary widely when the engine starts, possibly causing the engine to stall.

Moses  

[Bob S]

Thank you Moses, I have been working at this on and off and trying to get her going for the last year. bought her back in 78 with 1,638 mil on it. and I was ate wits end . Thank you I will let you know how I make out. thank you for so much info. Bob

[Moses Ludel]

Bob S...The possible ignition and solenoid issues came up when I reflected on the starter solenoid and ignition distributor found in a stock Commando 258.  If the breaker point distributor has a lot of miles, that would be a focus.  Take a few photos of the distributor, with and without the distributor cap and rotor removed.  Post them here, I'll look the parts over.

Moses