Calling all 4.5 PFI and 4.9 PFI Owners
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HT4100 4.1, 4.5, 4.9 Discussion, Calling all 4.5 PFI and 4.9 PFI Owners in Cadillac Engine Technical Discussion; It seems that around 60-70k miles these cars develop a condition where they begin to run rich. Post your issues ...
  1. #1
    awadecki is offline Cadillac Owners Enthusiast
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    Calling all 4.5 PFI and 4.9 PFI Owners

    It seems that around 60-70k miles these cars develop a condition where they begin to run rich. Post your issues and your solutions here to help everyone out, along with the mileage at which the issue was noticed.

    I had a 91 Deville with 68k miles that ran too rich and I finally traced it to a bad fuel pressure regulator. Replaced it and the oxygen sensor and it was fine.

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Replaced my o2 at 90xxx miles after the mileage dropped to around 9. I didnt get the car till it had over 80k on it so not sure what the previous owners did until then

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    lenny lincoln is offline Cadillac Owners Fanatic
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    after I change my O2 and fpr I tell ya what happens. I am only getting around 9 mpg in town driving

  5. #4
    gta3mobster's Avatar
    gta3mobster is offline Cadillac Owners Member
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Own a 4.5 MPFI. I suspect my car is running rich, among things. 190k miles and I'm not sure when the last tuneup was. It runs ok, though. I usually get around 14 mpg city. I have loose motor mounts so I have to drive the car around carefully (I let off almost all the way once the tranny switches gears. Goes buttery smooth then)

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    If 14 seems low for your drive look into possible changing the o2 sensor, my car ran fine, just rich. The o2 doesnt necessarily affect driveabality a noticeable amount

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    Pckstude is offline Cadillac Owners Member
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Gents,,, I have a 4100 in a '84 deville, with just over 65k on it and it has almost no acceleration. Actually no or little acceleration from a stop. The plugs, plug wires, dist. cap and rotor have been changed, along with the fuel filter, which was pretty much plugged. So I thought that perhaps that might have straightened things out a bit. The car sat in a garage for 7yrs having never been run all that time. That means that I am currently running on 7yr old gas. BTW I'm not on the road with it yet except for a few short miles to see how it runs.
    Thanks to this thread I will now check my fuel pressure regulator, and check my codes for an o2 sensor. Any other ideas would be appreciated. thanks

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Quote Originally Posted by Pckstude View Post
    Gents,,, I have a 4100 in a '84 deville, with just over 65k on it and it has almost no acceleration. Actually no or little acceleration from a stop. The plugs, plug wires, dist. cap and rotor have been changed, along with the fuel filter, which was pretty much plugged. So I thought that perhaps that might have straightened things out a bit. The car sat in a garage for 7yrs having never been run all that time. That means that I am currently running on 7yr old gas. BTW I'm not on the road with it yet except for a few short miles to see how it runs.
    Thanks to this thread I will now check my fuel pressure regulator, and check my codes for an o2 sensor. Any other ideas would be appreciated. thanks
    Siphon the tank?
    lol, I,m not sure how well gas holds up, 7 years seems like a long time though. I recently tried seafoam in my 4.9 and it made a bit of a difference when I ran it through the gas tank and into the throttle body. You may just need new gas/oil and drive it for a bit to get it all warmed and loosened up.

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    lenny lincoln is offline Cadillac Owners Fanatic
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    my O2 didnt throw codes and didnt help. Working the FPR with a mightyvac and WD40 smoothed out my idle and boosted my mpg. I also black taped the FPR hose so it was a tighter fit. I am now getting 28/13 mpg after a full tune up

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Quote Originally Posted by lenny lincoln View Post
    my O2 didnt throw codes and didnt help. Working the FPR with a mightyvac and WD40 smoothed out my idle and boosted my mpg. I also black taped the FPR hose so it was a tighter fit. I am now getting 28/13 mpg after a full tune up
    Did your fpr look to be leaking at all? My idle cleaned up after using seafoam, still a bit rough at times but my FPR doesnt show leaks, my mileage is pretty good though.

    Can you go abit more indepth on what you mean by working the fpr with wd-40 and a vacumm?

  11. #10
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    The HT4100s don't have an external fuel pressure regulator. It's included as part of the throttle body--and rarely goes bad. This issue largely concerns the PFI 4.x engines.

    By the way, the FPR doesn't have to leak to be bad. The diaphram can just be stuck--not punctured.

  12. #11
    Pckstude is offline Cadillac Owners Member
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Behind bars,,, I've tried several different ways to siphon or other wise get that bad gas out. Guess I'ld make a poor gas theif. lol Even the other day when I changed the fuel filter, I tried hooking up a hose to the downside of the new filter and turned the ignition on figuring that the pump in the tank would do the trick. No dice. It only pumped for a brief second or two when I first turned on the ignition switch. That seemed king of odd to me, but who I am to argue? lol So believe me. I've tried. The only option I see now is to just fill it. A can of seafoam has already been added. Still open to suggestions.

  13. #12
    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Quote Originally Posted by Pckstude View Post
    Behind bars,,, I've tried several different ways to siphon or other wise get that bad gas out. Guess I'ld make a poor gas theif. lol Even the other day when I changed the fuel filter, I tried hooking up a hose to the downside of the new filter and turned the ignition on figuring that the pump in the tank would do the trick. No dice. It only pumped for a brief second or two when I first turned on the ignition switch. That seemed king of odd to me, but who I am to argue? lol So believe me. I've tried. The only option I see now is to just fill it. A can of seafoam has already been added. Still open to suggestions.

    I've never siphoned either lol. If you have already done some work then I would just try and drive the rest of that gas out, or fill with some fresh to try and dilute it some. Personally I dont think I would do much more work till I was certain the gas wasnt the culprit to the problems.

    Maybe someone will have some good ideas, I dont recall, but did you clean the egr tubes beneath the throttle blades? The procedure is talked about alot on here and is pretty simple.

  14. #13
    lenny lincoln is offline Cadillac Owners Fanatic
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    I threw a can of seafoam into the tank and drove the car 5 miles , parked car on slanted driveway only to have the seafoam foul new plugs the following morning. 1 hr wasted time , new ignition module = $20, another 2hrs changing plugs and pulling lower breathing hose off tank to siphion the gas out, Tried the fuel pump tank siphioning method mentioned earlier got the same results. My take on seafoam, Freaking crap! cost me too much, keep it out of your car. snake oil! I will use ATF threw the carb followed by water next time.

  15. #14
    gta3mobster's Avatar
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    For those with poor MPG, try the lucas fuel system cleaner. It's a really thick (like warm syrup) gasoline additive. I picked up a jumbo bottle for $8, which is enough to treat 100 gallons.

    I didn't really pay attention to see if I got any gains, but I did notice the car was running smoother. It started going to the 70 mpg when coasting a lot more too.

  16. #15
    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Calling all 4.5 PFI and 4.9 PFI Owners

    Weird, I had no trouble with the seafoam. My driveway is slanted so I backed into the level street, put a can of seafoam into the throttle body with the engine running, then killed the engine and let it set for 20 mins while I cleaned the inside of the TB, Fired the car up and circled the block a few times to burn tit all off, then threw another bottle into the tank when I got gas and didnt have any problems.

    I have read though that you can also use water through the TB, but since its not combustible like seafoam I would worry more about hydrolocking. It was hard to get restarted the first time after seafoam in the TB, but didnt messanything up

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