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I have a 87’ jeep 4.2L in-line 6 and ive been having a few…

I have a 87’...

I have a 87’ jeep 4.2L in-line 6 and ive been having a few problems with timing and idling too high, the specs from the mech manual call for idle rpms from 500-600 and it’s sitting at 2000 but right now when I turn off the ignition it just idles for about 2-3min then back fires then turns off

Mechanic's Assistant: How many miles are on the car? What size is the engine?

Odometer reads just over 187000 but at 171000 there was an engine re build and replaces with new parts, and 4.2L inline 6

Mechanic's Assistant: What is the model/year of your Jeep?

YJ/ 1987

Mechanic's Assistant: Are you fixing your vehicle yourself? What have you tried so far?

Yes and no, so clutch, Weber 2 barel carborator, distributor are all brand new I havnt tried adjusting timing just yet and tightness on the carb. I was doing some research before I messed with any of thet yet

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

The electrical harness dropped down on the exhaust manifold and I re wired that but that’s it

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Answered in 13 minutes by:
3/19/2018
Matt
Matt, Engineer
Category: Car
Satisfied Customers: 22,908
Experience: Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
Verified

Hello and welcome to Just Answer

This could be an airleak after the carb, any air dragged in here isn't 'seen' by the system and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propaneor brake cleaner around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

1 vacuum leak path that you won't find with the above test is if the brake servo is leaking

so try clamping off servo hose to see if this has any effect

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Matt
Matt, Engineer
Category: Car
Satisfied Customers: 22,908
Experience: Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
Verified
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Matt
Matt, Engineer
Category: Car
Satisfied Customers: 22,908
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Experience: Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.

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