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post #1 of (permalink) Old 02-15-2002, 06:10 PM Thread Starter
 
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solved: 258 Low speed miss

( EDIT: it was a ported Vacuum leak in the EGR valve)

258 Low speed miss
My 258 has a miss at 1k to 2k. It idles great. Super smooth. For example if I put it in neutral and try to hold a steady RPM above idle it misses. PS The engine is bone stock. I live in CA so a computer bypass is not an option, not that I suspect the ECM.

Please help, I donít know what the most likely causes are. I suspect the ignition, but Iím hoping that someone out here has a recommendation.


1989 jeep YJ,4.2L,5spd
"Roger-dodger, putt putt"
[img] http://members.aol.com/_ht_a/gouldvo...p/ltl_jeep.gif [/img]
http://hometown.aol.com/gouldvolts/jeep/jeep.html
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post #2 of (permalink) Old 02-15-2002, 07:14 PM
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Re: 258 Low speed miss

If it's a regular miss, it could be secondary ignition. Does it miss badly if you give it a lot of throttle at moderate speed?

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post #3 of (permalink) Old 02-15-2002, 08:34 PM Thread Starter
 
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Re: 258 Low speed miss

no.
mild miss not a bad one. its only noticeable with light steady throttle. it drives fine.

it is very noticeable in neutral. it idles fine, but give it a light steady press on the gas to get it to 1.3K RPM and it will be a little reluctant to get to 1.3k and miss while trying to maintain 1.3K RPM. A quick hard press will get the engine to rev.


1989 jeep YJ,4.2L,5spd
"Roger-dodger, putt putt"
[img] http://members.aol.com/_ht_a/gouldvo...p/ltl_jeep.gif [/img]
http://hometown.aol.com/gouldvolts/jeep/jeep.html
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post #4 of (permalink) Old 02-15-2002, 09:13 PM
 
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Re: 258 Low speed miss

sounds like the throttle body has a dirty injector (providing your 89 is injected) if it is carbed than you got the typical carter problems

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post #5 of (permalink) Old 02-15-2002, 09:29 PM Thread Starter
 
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Re: 258 Low speed miss

This is a carter BBD jeep, but it idles great!!! and that is what most people complian about. right?

1989 jeep YJ,4.2L,5spd
"Roger-dodger, putt putt"
[img] http://members.aol.com/_ht_a/gouldvo...p/ltl_jeep.gif [/img]
http://hometown.aol.com/gouldvolts/jeep/jeep.html
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post #6 of (permalink) Old 02-16-2002, 08:51 AM
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Re: 258 Low speed miss

Your miss is almost certainly due to carburetor problems; most likely blockage in the mid-range circuit. Your best bet would be to get a rebuild kit and dive in. The kits are cheap, and the rebuild easy. You've nothing to lose but a sawbuck and a few hours.

Be sure to note the setting on the needle valve whan you're disassembling it. When you have everything apart, blow out all of the passages with compressed air. Be sure to check the weight of the float with a postage scale. The foam ones soak up gasoline over the years, and the brass ones sometime get a pinhole leak.

Put it back together according to the specs in the rebuild kit; don't try to make any 'improvements' in the process. And put in new fuel and air filters while you're at it. Almost certainly the problem will be gone when you get it back together.

My carb goes wonky about once a year. I never see anything obviously wrong with it when I open it up, but it's always better afterwards. One of life's little mysteries I've given up trying to understand.



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post #7 of (permalink) Old 02-17-2002, 12:21 AM Thread Starter
 
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Re: 258 Low speed miss

what about a vacuum leak? how can I rule out a vacuum leak?

1989 jeep YJ,4.2L,5spd
"Roger-dodger, putt putt"
[img] http://members.aol.com/_ht_a/gouldvo...p/ltl_jeep.gif [/img]
http://hometown.aol.com/gouldvolts/jeep/jeep.html
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post #8 of (permalink) Old 02-17-2002, 08:18 AM
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Re: 258 Low speed miss

A can of spray oil like WD40 can help. Spray it around anything related to the intake system; carb base, intake manifold gaskets and so on. If the engine sucks in some oil and propellant it will temporarily change it's idle. Don't forget the vacuum hoses. Any hose you can't check to the end you can clamp closed and see if the problem changes.

Vacuum leaks usually are so small that they won't affect the engine once it's off idle. A leak big enough to affect it on the mid-range circuit should be big enough to hear.

There are several other possibilities like a bad EGR valve and bad PCV valve that are less likely to cause the problem, and more difficult to eliminate, which is why I suggest doing the other things first. Also I would guess that your engine has the electronic controls on the carburetor, about which I know nothing. Do a search for "nutter bypass" for info on that and see if it applies.



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post #9 of (permalink) Old 02-17-2002, 06:45 PM Thread Starter
 
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Re: 258 Low speed miss

The answer: Ported vacuum leak.

the EGR valve had a giant vacuum leak. the EGR valve is controlled by ported vacuum. I plugged the vacuum line going to the EGR valve and it worked allot better. I put my handy dandy sears vacuum pump onto the EGR valve and gave it a few squeezes. It couldn't hold vacuum.

this site was very helpful
http://home.sprynet.com/~dale02/vac-egr.htm


1989 jeep YJ,4.2L,5spd
"Roger-dodger, putt putt"
[img] http://members.aol.com/_ht_a/gouldvo...p/ltl_jeep.gif [/img]
http://hometown.aol.com/gouldvolts/jeep/jeep.html
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post #10 of (permalink) Old 02-17-2002, 07:14 PM
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Re: 258 Low speed miss

See, you're MORE than just another pretty face. Congratulations.

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