Stalling when revving down

MJLiFe

MJLiFe

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Hello all I’m new here and hope I can help some of you and vice versa. I have an 89 Comanche with a 4.0. The previous owner deleted most EVAP components which is giving me a headache. I have a stall issue when coming out of gear and engine is revving down. I cleaned throttle body, replaced plugs, wires, cap and rotor, fuel filter, air filter,and capped any existing vacuum lines. It also hesitates slightly when inducing throttle. Map sensor seems to be working fine because there is no misfire. Also battery sits at a healthy 12.5V and 14.7V when running. But still dies after revved up high just can’t keep steady idle. Any thoughts would be greatly appreciated thanks again.
 

Update. I narrowed it down and will be readjusting the TPS some time this week hoping that this will fix the issue I will update again when I’m done if there are any other ideas I would greatly appreciate the input. Thanks


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It idles around 1200. Plugs are not wet after stalling. Cleaned IAC and had no change. Have not measured the fuel pressure yet but will do that thanks so much.


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So you air/vac leak and not a fuel issue.
Check for an intake leak or a vac line leak.
Idle should be ~750 rpm


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Last edited:
Great thanks again this weekend I will check all vacuum lines. I already removed throttle body cleaned and replaced gasket. Will update on progress.


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Start it and let it idle. Take WD40 or carb cleaner and spray around intake area then all vac lines. Injectors and plugs. When the idle changes or shudders from the spray you found the leak. Post back if you done.

Of course these are flammable, so no open flame and don’t let the engine get hot.


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Gotcha thanks I will run that test tomorrow. As for the TPS I’m getting 4.98V going in and no reading on the signal side so also trying to track that issue. Again thanks and will update results tomorrow.


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Update.. The TPS was out of adjustment so I finished that which kept it from stalling letting off the throttle but still drops real low and shutters. Idle seems to be steady at around 800 when just sitting. No luck yet with the vacuum leak. Going to mess with the IAC tomorrow see if there’s any luck. Thanks again for all the help this far.


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It should idle 750-800. Shuttering could be anything from fuel pressure, fuel filter, upstream o2 sensor, plug, injector, temp sending unit, cps to ckp timing... id start with testing and cheap parts first like plugs.


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Update.. the idle is steady and doesn’t stall after releasing throttle but still dips really low. I’ll continue to troubleshoot this issue and update any other things I find out


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Also strange thing after changing plugs wires and distributor it blew some pretty good black soot from the exhaust. I know it will normally do some but this was thick and a decent amount. Just seeing if that triggers any thoughts. Thanks again.


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You he led the plugs in an earlier post. You said they were not wet. Were they black and sooty? Burning rich or burning oil will load the CAT. In order to clean it out some you could run it hard >3300 rpm kicking it up rapidly. If you are running rich at high idle and let off the pedal quickly you may be choking the engine with loaded gas and no air. Fuel pressure, stuck injector or either not gapped enough plugs or not hot enough spark.
If you want to check if a blocked CAT can be the issue remove the upstream O2 tie it up out of the way if you plan on driving with it out. You will(should) throw a code. If your CAT is blocked you should be throwing some codes.

After you put it back. Sea foam it. Start with a drizzle of water that will steam the engine. Do not let more than a fine mist in. Avoid hydro lock. Then follow with sea foam. The idle above 3300 rom helps rotate the valves. They are designed to float and rotate a bit at high RPMs.


Also were your plugs gapped properly?


Regards,
JPNinPA

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Last edited:

Yes they were black and soot covered but not wet. Also one of the plugs terminals came out with the old wire. Mind I bought it like this. It wasn’t misfiring at all but you can definitely tell the difference in power. I’m guessing most of the soot was being burnt out of that cylinder from the un burnt mixture in there. The old plugs were out of gap. Now are set to .035 like manufacture recommended. I kept with the copper core to allow the burn to stay the same. I am going to check the cat like you are saying. The rpms don’t drop initially letting off the gas anymore comes down to 1000 then settles at 800 then for some reason dips and shutters a little after that.


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That is almost normal. Id just run some injector cleaner and do the steam then sea foam and see where you are. At this point it may just need to clean up and relearn its new norm.
Other than this, it still sounds like it may be running rich Or the IAC is slow. Either way time will tell. In a few hundred miles you could recheck the plugs. Sooty is rich or oil burning clean would be the air side like the IAC or small vac leak.


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