'94 Deville4.9L idles rough !
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View Poll Results: 4.9L rough idle

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HT4100 4.1, 4.5, 4.9 Discussion, '94 Deville4.9L idles rough ! in Cadillac Engine Technical Discussion; New: Spk plugs, wires, Dist. cap, rotor, coil,TPS, Vaccum lines/connectors, filters, Mobile 1, Only Chevron Premium. Performed Top Engine Cleaner ...
  1. #1
    2Clown2 is offline Cadillac Owners Member
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    '94 Deville4.9L idles rough !

    New: Spk plugs, wires, Dist. cap, rotor, coil,TPS, Vaccum lines/connectors, filters, Mobile 1, Only Chevron Premium.
    Performed Top Engine Cleaner as directed.
    Still it "misses", or rough idles. It Is perfectly smooth at startup for about a minute. Also, it is better when in gear and A/C is on.
    My guess is some hidden vacc. leak (idles @ 40-41)
    or plug wires arking.

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  3. #2
    Elvis's Avatar
    Elvis is offline Cadillac Owners Master
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    Re: '94 Deville4.9L idles rough !

    Don't know this for sure, but it seems like whenever something like this comes up, it turns out to be some 02 sensor failing somewhere.

    The cold start info you gave is what points me in that direction.

  4. #3
    2dfx's Avatar
    2dfx is offline Cadillac Owners Fanatic
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    Re: '94 Deville4.9L idles rough !

    Next things you might wanna check are the ignition timing, and the distributor itself.

    O2 sensor would not cause a miss and would set a code. Try running diagnostics and check for codes.

  5. #4
    Stoneage_Caddy's Avatar
    Stoneage_Caddy is offline Cadillac Owners 10000+ Posts
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    Re: '94 Deville4.9L idles rough !

    anyone see that he replaced his TPS ? wonder if hes done idle relearn procedure?

    i also wonder about his EGR tubes under the throttle plate being clogged or worse his egr valve not working

    i also wonder about his fuel pressure at idle , wonder what the spec is there

    looks like me and 2dfx have another sibling here

  6. #5
    2Clown2 is offline Cadillac Owners Member
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    Re: '94 Deville4.9L idles rough !

    Quote Originally Posted by 2Clown2
    New: Spk plugs, wires, Dist. cap, rotor, coil,TPS, Vaccum lines/connectors, filters, Mobile 1, Only Chevron Premium.
    Performed Top Engine Cleaner as directed.
    Still it "misses", or rough idles. It Is perfectly smooth at startup for about a minute. Also, it is better when in gear and A/C is on.
    My guess is some hidden vacc. leak (idles @ 40-41)
    or plug wires arking.

    Fixed rough idle. Capped ALL of the vaccum outlets except the Vacc. Sensor, and put a new rotor cap on. Now it's "Pinging" sometimes.
    Gas is Chevron premiun and a bottle of O'Riely octaine booster. Can anybody tell me why it's pinging ?
    No codes. One O2 sensor cycles a lot faster. Idle re-learn proceedure done.

  7. #6
    Stoneage_Caddy's Avatar
    Stoneage_Caddy is offline Cadillac Owners 10000+ Posts
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    Re: '94 Deville4.9L idles rough !

    probably the egr system , i see alot of 4.9 pining issues resoved with the cleaning fo the egr tubes under the throttle plates and if it still pings, replecment of the egr valve , there is a cadillac "how to" message board but i dont have the link on me , the caddy 4.9 doesnt have a knock sensor so the ecu doesnt know if its pingin or not , instead it comes up with a crude idea on pinging based on readings from sevreal sensors , it uses the egr system to keep the engine from pinging

  8. #7
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    Re: '94 Deville4.9L idles rough !

    Quote Originally Posted by 2Clown2
    New: Spk plugs, wires, Dist. cap, rotor, coil,TPS, Vaccum lines/connectors, filters, Mobile 1, Only Chevron Premium.
    Performed Top Engine Cleaner as directed.
    Still it "misses", or rough idles. It Is perfectly smooth at startup for about a minute. Also, it is better when in gear and A/C is on.
    My guess is some hidden vacc. leak (idles @ 40-41)
    or plug wires arking.

    I recently had a COMPLETE tune up. ran well for a few days and then started stalling and bucking.

    Popped the hood and looked around. Noticed the garage hadn't hooked up a vac line to the cannister.

    Simple fix for me. Hopefully for you too.

  9. #8
    ga_caddy is offline Cadillac Owners Member
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    Mine is doing the same thing. I guess I need to get in there and look at things. I honestly don't remember it acting that way until I cleaned the TB and did the idle relearn.

    Maybe I need to blank the computer memory overnight and do the relearn?

  10. #9
    2Clown2 is offline Cadillac Owners Member
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    Re: '94 Deville4.9L idles rough !

    Quote Originally Posted by ga_caddy
    Mine is doing the same thing. I guess I need to get in there and look at things. I honestly don't remember it acting that way until I cleaned the TB and did the idle relearn.

    Maybe I need to blank the computer memory overnight and do the relearn?

    Found it ! Remove the vacc line to the cruise, evap solenoid, vaccum reservoir, and cap the 1/4' vaccum pipe to the Throttle Body. It's the one on the psngr side to the upper rear. It angles up slightly. (See Attachment) If this smoothes out the idle then thats where the problem lies. I found several rubber hoses that were carbonized! Likely the evap hose !

  11. #10
    2Clown2 is offline Cadillac Owners Member
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    Re: '94 Deville4.9L idles rough !

    Quote Originally Posted by 2Clown2
    Fixed rough idle. Capped ALL of the vaccum outlets except the Vacc. Sensor, and put a new rotor cap on. Now it's "Pinging" sometimes.
    Gas is Chevron premiun and a bottle of O'Riely octaine booster. Can anybody tell me why it's pinging ?
    No codes. One O2 sensor cycles a lot faster. Idle re-learn proceedure done.
    Found it ! Removed the vacc line to the cruise, evap solenoid, vaccum reservoir, and capped the 1/4' vaccum pipe to the Throttle Body. It's the one on the psngr side to the upper rear. It angles up slightly. (See Attachment) This smoothed out the idle, so thats where the problem lies. I found several rubber hoses that were carbonized! Likely the evap hose was the one!

  12. #11
    ga_caddy is offline Cadillac Owners Member
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    Re: '94 Deville4.9L idles rough !

    Quote Originally Posted by 2Clown2
    Found it ! Removed the vacc line to the cruise, evap solenoid, vaccum reservoir, and capped the 1/4' vaccum pipe to the Throttle Body. It's the one on the psngr side to the upper rear. It angles up slightly. (See Attachment) This smoothed out the idle, so thats where the problem lies. I found several rubber hoses that were carbonized! Likely the evap hose was the one!

    I figured that might be it. I checked all the vac lines today and the one to the cruise/ac/evap is just about gone. I bought some new vac line and an EGR Valve gasket this evening. I plant to remove the EGR and clean everything there then replace the vac lines.

    Thanks for the help!!

    Now I just have to figure out what to do with the heater core/evap box that is falling apart.

  13. #12
    2Clown2 is offline Cadillac Owners Member
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    Rough Idle/Vaccum leak/Evap line to gas tank

    Traced the vaccum leak to the line from the gas tank to the Evap canister. Looks good up arround the cannister so it must be either under the car or where it goes into the tank. Do I need to drop the tank? Anyone had this? My Alldata diagrams aren't satisfying. (See Attach.)

  14. #13
    2Clown2 is offline Cadillac Owners Member
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    Re: '94 Deville4.9L idles rough !

    Quote Originally Posted by wildoaf
    I recently had a COMPLETE tune up. ran well for a few days and then started stalling and bucking.

    Popped the hood and looked around. Noticed the garage hadn't hooked up a vac line to the cannister.

    Simple fix for me. Hopefully for you too.

    That was it! Been looking for this leak for too long! Mine must be either under the car or at the gas tank. What finally proved it for me was that I've never heard vaccum release when opening the gas cap.
    Thanks.

  15. #14
    ga_caddy is offline Cadillac Owners Member
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    Re: '94 Deville4.9L idles rough !

    Quote Originally Posted by 2Clown2
    That was it! Been looking for this leak for too long! Mine must be either under the car or at the gas tank. What finally proved it for me was that I've never heard vaccum release when opening the gas cap.
    Thanks.
    The Vapor line from the tank to the canister should not have vacuum. It simply goes into the Charc Canister where the vapors and any condensed fuel are stored until the canister purge Sol opens to allow vacuum to purge the vapors. If you are getting vacuum in your tank you need to clean the tank vent valve. This is a hose that is attached to the Canister and then is clamped to the cooling fan support. You take the cap off the hose, blow thru the cap to make sure it's clean and put it back.

    You should also test the Charc Canister and Purge Sol for proper operation.

  16. #15
    Night Wolf's Avatar
    Night Wolf is offline Cadillac Owners 10000+ Posts
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    my 4.9 can be rough at an idel.......but as soon as the rpm comes it, she smooths out and purrs really nice......it's just one of those 4.9 things ya know? since there isn't much that goes wrong with these enignes, we gotta have something.....it gives the car that nice classic muscle car rumble at idel attitude

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