Vacuum reading
Thread Starter
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Freshman User
Joined: Jul 2003
Posts: 42
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From: Ontario, Canada
Vacuum reading
Hello there,
I was just wondering what intake mainfold vacuum reading 2.9L engine has while idleing?
Mine has 18" Hg, is that a bit too low? What should it be according to the specs?
Thanks
Tom
I was just wondering what intake mainfold vacuum reading 2.9L engine has while idleing?
Mine has 18" Hg, is that a bit too low? What should it be according to the specs?
Thanks
Tom
Postmaster
Joined: Dec 2001
Posts: 3,742
Likes: 2
From: UT
"Correct" vacuum reading is dependent on altitude. AT my altitude (~5000 ft), I get 15-16 in Hg at idle when the engine is running properly. If the absolute pressure in the manifold stays the same, the gauge reading should change by ~1 in Hg per 1000 feet in altitude. I would expect a sea level reading to be ~20 in Hg.
Thread Starter
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Freshman User
Joined: Jul 2003
Posts: 42
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From: Ontario, Canada
Thanks Guys. The reason I was asking this is because I thought it was a bit on a low side. Also, sometimes I find that the vacuum reading drops to about 14" Hg for a second or two. The engine sounds like it wants to die. This only happnes once in a blue moon (very very seldom), when the engine is cold, but it does happen. Can any one tell me what that might be? ( I know this happens becouse I have a vacuum gauge installed in my truck)
Thanks once again,
Tom
Thanks once again,
Tom
Postmaster
Joined: Dec 2001
Posts: 3,742
Likes: 2
From: UT
Vacuum reading by itself isn't enough to diagnose this engine stumble. As noted, the low vacuum reading is likely a result of the engine stumbling rather than the cause of the engine stumbling.
Lots of things need to be looked at.
1) Tune-up state? How many miles on spark plugs, wires, air filter, fuel filter, etc.? Any other maintenance issues that need to be resolved?
2) Do the EEC-IV diagnostic tests indicate anything? Especially important if you have an '86 or '87 2.9, because those motors didn't come with a functional check engine light wired into the PCM. Anecdote: Mine had a cold engine stumble that turned out to be a bad O2 sensor. EEC-IV tests led me to replace the O2 sensor.
3) Fuel pressure/fuel delivery issues? Check fuel pressure. Is it dropping off/spiking when the engine stumbles? Are any of the fuel injectors not firing when the engine stumbles?
Complicating the diagnosis is the fact that it's an intermittant problem. The only "consistency" you've indicate to it is it occurs only when the engine's cold.
Lots of things need to be looked at.
1) Tune-up state? How many miles on spark plugs, wires, air filter, fuel filter, etc.? Any other maintenance issues that need to be resolved?
2) Do the EEC-IV diagnostic tests indicate anything? Especially important if you have an '86 or '87 2.9, because those motors didn't come with a functional check engine light wired into the PCM. Anecdote: Mine had a cold engine stumble that turned out to be a bad O2 sensor. EEC-IV tests led me to replace the O2 sensor.
3) Fuel pressure/fuel delivery issues? Check fuel pressure. Is it dropping off/spiking when the engine stumbles? Are any of the fuel injectors not firing when the engine stumbles?
Complicating the diagnosis is the fact that it's an intermittant problem. The only "consistency" you've indicate to it is it occurs only when the engine's cold.
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Freshman User
Joined: Jul 2003
Posts: 42
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From: Ontario, Canada
I know, it almost doesn't happen, that's how often the engine stumbles. But thanks for your help. I know its a bit difficult to figure out what is wrong with it.
I keep good maintnance record on my cars/trucks
Thanks
Tom
I keep good maintnance record on my cars/trucks
Thanks
Tom
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Elder User
Joined: May 2003
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From: Mobile, AL
Check the connections at your external fuel pump, mine were loose and dirty, causing a "stumble" every now and then. I repaired the connections and replaced the factory mount with two 2.5" x 3.5" u-bolts bolted through the frame. Works like a charm and you can inspect the pump easier now.
It may not be the cause of your "stumble" but it was mine. Good luck with it
It may not be the cause of your "stumble" but it was mine. Good luck with it
