'90 DeVille diagnostic mystery
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Cadillac DeVille 1985 to 2005 including:
1985-1992 Fleetwood, 1993 Sixty Special, All FWD Forum Discussion, '90 DeVille diagnostic mystery in Past Cadillac Vehicle Discussion; 4.5L transverse V8 - 70k miles In the past couple of months we've replaced injectors (new OEM), fuel pump (and ...
  1. #1
    Notaguru is offline Cadillac Owners Fanatic
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    '90 DeVille diagnostic mystery

    4.5L transverse V8 - 70k miles
    In the past couple of months we've replaced injectors (new OEM), fuel pump (and wiring), cap/rotor, plugs (PITA), EGR valve, ignition wiring, fuel filter, air filter, and cleaned throttle body. Fuel pressure regulator was checked and functions properly.

    NOW - car starts and runs fine, but after five minutes it runs rough and stays that way. No power regardless of throttle position - it barely makes 40mph.

    Clue? We found a lot (6-8 ounces?) of water in the muffler yesterday. It drained when the car tipped on the rack...

    Cat converter? Today we'll disengage the front end of the exhaust and see if the problem goes away.

    Can you think of anything else that is temp sensitive or that might be causing the problem?

    THANKS!

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    Manic Mechanic is offline Cadillac Owners Fanatic
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    Re: '90 DeVille diagnostic mystery

    Don't drop the exhaust yet, unplug the O2 sensor. If it runs better you have a bad O2 that's biasing the fuel trim once it reaches closed loop. If it makes no difference unplugged remove it to check if it's caused by exhaust back pressure, hey it's already unplugged and it's a lot easier than dropping the exhaust pipes.

    Vernon

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    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: '90 DeVille diagnostic mystery

    Clue? We found a lot (6-8 ounces?) of water in the muffler yesterday. It drained when the car tipped on the rack...
    Unplug the weep holes in the bottom of the muffler.

    If we where betting, I'd put my money on a clogged CAT.

  5. #4
    Notaguru is offline Cadillac Owners Fanatic
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    Re: '90 DeVille diagnostic mystery

    We replaced the O2 sensor because a more sophisticated code reader said to, and it cost only $21 with my NAPA discount. Idled/etc. to reprogram and it was smoother than before. A definite improvement!

    However, under load it flaked out again. Tomorrow morning we'll put it back on the rack and go after the exhaust. We're lucky to have access to the auto hobby shop at a local base.

    @#%$@!!

  6. #5
    Notaguru is offline Cadillac Owners Fanatic
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    Re: '90 DeVille diagnostic mystery

    DIAGNOSIS:

    With a die grinder and a small abrasive disc, I cut a 1" x 2" hole in the exhaust pipe between the cat converter and the muffler. Two minutes' work.

    No change in performance.

    I then cut a similar hole in front of the cat converter.

    Car runs like a Cadillac again.

    SOLUTION:

    Replaced cat converter, welded holes, PROBLEM SOLVED.

    Thanks to the gurus here - yet again!

  7. #6
    Notaguru is offline Cadillac Owners Fanatic
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    Re: '90 DeVille diagnostic mystery

    PROBLEM SOLVED - NEW PROBLEM

    Well, the car does run better with the cat converter replaced, and while the exhaust was off we scoped the muffler and saw that it was not blocked. But the car is not right.

    It's running RICH - so much so that gas can be smelled. And that means it could quickly wreck the new cat converter. And, it seems to bog down somewhat. So - it's far better, but still wrong.

    Remember, the EGR valve, injectors, plugs, fuel pump, filter, and a lot more have just been replaced. We just inspected everything, and there's nothing left unconnected, no vacuum lines loose. No code appears. What could it be now?

    @#&!!

  8. #7
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    MoistCabbage is offline Cadillac Owners 10000+ Posts
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    Are you SURE the FPR is good?

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    Re: '90 DeVille diagnostic mystery


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    Notaguru is offline Cadillac Owners Fanatic
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    Re: '90 DeVille diagnostic mystery

    The reason we replaced the fuel pump (what a PITA!) was that pressure was measured at 33psi, 10# low, and a garage tested the FPR and pronounced it okay. The new pump and wiring gave us the prescribed pressure, so we believe that the FPR works properly.

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    stoveguyy is offline Cadillac Owners Connoisseur
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    Is there a way to hook up car to scan tool and see if injector pulse widths are right on? U know fuel pressure is good, new injectors. O2 readings say oxy value is good? Bad PCM?

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    Re: '90 DeVille diagnostic mystery

    Quote Originally Posted by Notaguru View Post
    The new pump and wiring gave us the prescribed pressure, so we believe that the FPR works properly.
    That's irrelevant. You can have good pressure with a leaky diaphragm. If the diaphragm is leaking, it is putting unmetered fuel into the intake manifold thus causing a rich fuel condition. Just pull the vacuum line off the FPR at idle and look for any sign of fuel at the nipple. There should be none.

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    Notaguru is offline Cadillac Owners Fanatic
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    Re: '90 DeVille diagnostic mystery

    Thanks. The shop lent us a tester, which says the fpr is ok. I also did your test (pulling the vacuum line) - no gas evident. We're suspecting a stuck injector.

  14. #13
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    Re: '90 DeVille diagnostic mystery

    Does the 90 use multiple O2 sensors? Did your tester show both if equipped? Not sure if ODBI has capabilities to show desired and actual injector pulse times. Just a thought but have you checked/changed the o rings on the FPR?
    I re-read your posts and about the only thing Not replaced is the FPR. Might consider doing it, wouldn't be the first part that tested good but was junk.
    It would almost have to be multiple injectors sticking for you to smell it so quickly or one completely broken letting fuel dump in.

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    Re: '90 DeVille diagnostic mystery

    What does your fuel pressure do when you step on the accelerator?

  16. #15
    Notaguru is offline Cadillac Owners Fanatic
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    Re: '90 DeVille diagnostic mystery

    Quote Originally Posted by dennis93coupe View Post
    What does your fuel pressure do when you step on the accelerator?
    Steady at about 43psi. We taped the meter to the hood for the drive.

    We'll do more checking today:

    1. Pull the vacuum line from the FPR to check for gasoline.
    2. Disconnect one injector at a time to see if it makes a difference.
    3. Re-check all 19,873 connections (vacuum, air, electrical, mechanical) under the hood.
    4. --- if nothing is found, it'll go into the shop for $$

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