Engine Stumble/exhaust leak when cold? - Defender Source
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  #1  
Old December 21st, 2015, 01:13 PM
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Stephan Laputka
1995 D-90 SW
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Engine Stumble/exhaust leak when cold?

1995 NAS D-90, 4.6. Has this strange issue that is primarily tied to the ambient air temperature/ engine temp. When first starting (so totally cold) it will fire up fine, idle fine but when you rev it, it stumbles like it's down a cylinder and sounds like it has a bad exhaust leak. Once it warms up, revs fine, exhaust sounds fine, no stumble, idle is still fine, pulls strong.

I did some searching and there are numerous threads about engine stumbles but those are always associated with a rough idle or intermittent dying. I have none of that.

Recent changes: new temp sender, new ICV, and the dealer recently adjusted the base idle (despite be telling them it needs a new ICV and to not touch the idle). I checked the leads, sprayed around for an air leak, checked the usual connections and all seems fine.

I thought my best chance is a timing issue but isn't it backwards? Usually you set the timing and issue would show up while hot if the advance wasn't working? If the advance was set too far after TDC could it stumble while cold despite everything else working properly? I believe I set it to 12 after TDC.

Appreciate suggestions. Thanks
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  #2  
Old December 23rd, 2015, 02:45 PM
Departing90
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Sasha
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Is your engine a "4.6" with a distributor? Your symptoms sound like typical mass air flow sensor issues. Look for leaks, remove and clean with carb cleaner spray "watch u tube videos" and reassemble. If no luch find a known good maf sensor to test. Its not your timing unless your revving to redline and symptoms appear. Timing issues generally show tye same symptoms at all heat ranges and rpms.

------ Follow up post added December 23rd, 2015 02:52 PM ------

If you have a distributor also check tye vaccume advance "its a disc / dome shape gismo" on the side of tye distributor by taking off tye hose and checking suction by placing tye tip of your finger on the outlet. If you dont feel suction tye vaccume advance is bad. Check for leaks in tye hose between the vaccume advance and intake.

Take off tye distributor cap and check play in the rotor. If you have play tye advance mechanism underneath tye rotor cover is bad.
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  #3  
Old December 23rd, 2015, 03:08 PM
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Chris Davis
94 NAS D90 6.2LS
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Also, what distributor are you running? Does it have a ballast resister such as many of the Mallorys use? If so, you may want to check that out. Mine had some stumbling issues once and I traced it back to that. To me, though, it still sounds like an IAC issue and/or vacuum leak. I know that you said it was replaced---but it still sounds like that particularly on the cold start where that is in use.
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  #4  
Old December 23rd, 2015, 03:12 PM
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Ed
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Does it happen all the time, or only when it rains?
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  #5  
Old December 24th, 2015, 01:13 PM
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Stephan Laputka
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I took a second after reading the responses and I think I might have found the culprit. The Vaccum advance is dead. Sucked on the hose and it pulls air so that's not working. It's not the line so the diaphragm must be shot. Curious why that would be the issue though. Doesn't the advance mainly deal with the higher rpms?
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  #6  
Old December 24th, 2015, 01:32 PM
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shayne young
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No.. Vacuum advance deals with throttle, mechanical deals with engine speed..
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  #7  
Old December 24th, 2015, 06:21 PM
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Stephan Laputka
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So theoretically that could be the problem since the stumble only happens when throttle is applied?
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