compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...
Cadillac
 

Cadillac Forums | Help Us Help You | Advertise | Cadillac Parts | Cadillac News | Cadillac Classifieds / (Old System)

Cadillac Technical Archive | Cadillac Dealers | Cadillac Reviews | Cadillac Dealer Reviews | Cadillac Vendors

CadillacForums.com is the premier Cadillac Forum on the internet. Registered Users do not see the above ads.
+ Reply to Thread
Results 1 to 5 of 5
HT4100 4.1, 4.5, 4.9 Discussion, compression test on 1990 4.5L ? constant misfire, idling, under load, coasting... in Cadillac Engine Technical Discussion; Just acquired a 1990 Deville with a 4.5L I have a chilton manual which covers the car, but does not ...
  1. #1
    devillekelly is offline Cadillac Owners Member
    Automobile(s): 1990 deville
    Join Date
    Nov 2013
    Age
    53
    Posts
    1

    compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    Just acquired a 1990 Deville with a 4.5L I have a chilton manual which covers the car, but does not give a number for compression cylinder pressure. Can I run a compression test in the usual manner? And what is a good ballpark number? It has a bad misfire even though the PO said a "tuneup" was just done (to the tune of $500, and the "mechanic" said it was in fine shape now). There is no smoke coming from the car and the oil looks clean and normal. Thought I'd start by checking for a bad plug(s) or wire(s) and then do a compression test... if I don't find the culprit, where shall I look next?
    Thanks in advance for any constructive input!
    Kelly

  2. Remove Advertisements
    CadillacForums.com
    Advertisements
     

  3. #2
    onebadsuv is offline Cadillac Owners Member
    Automobile(s): Northstar plus others
    Join Date
    Aug 2013
    Posts
    2

    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    Greetings - 1990 4.5L Deville engine has the port fuel infection (injection) system . When You pull the plugs , check to see which are black or sooty looking , as they indicate which cylinders misfire ... bad Injectors foul plugs quickly

  4. #3
    cadillac_al is offline Cadillac Owners Connoisseur
    Automobile(s): Cadillac 78 Seville, 78 Coupe de Ville, 92 Sedan de Ville
    Join Date
    Feb 2007
    Location
    Maine
    Age
    52
    Posts
    2,710

    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    Most engines are in the 120-150 compression range. A $500 tune up should include AC Delco spark plug wires or some other high quality name like MSD or Moroso. If they are just cheap wires they will always be suspect. I always run some Techron in any new used car i buy. It is the only snake oil I ever use or recommend. It's always nice to verify plug gaps too because there is a lot of misprints out there. I think these cars are supposed to be .045"

  5. #4
    carnut is offline Cadillac Owners Master
    Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
    Join Date
    Feb 2003
    Location
    california
    Age
    66
    Posts
    5,699

    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    I would unplug each injector first to locate the misfiring cylinder. Just like pulling one plug wire at a time to localize the cylinder. Compression tests are done AFTER electrical culprits are ruled out! I'm thinking faulty injector or cracked spark plug.

  6. #5
    ht4100's Avatar
    ht4100 is offline Cadillac Owners Member
    Automobile(s): 88 Eldorado Biarritz, Copper Beige
    Join Date
    Nov 2005
    Location
    Lincoln, Nebraska
    Posts
    41

    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    my 4.9 had 145-150 psi, You could do a compression to rule that out if good. If a tune up has been done, plugs, wires, cap and rotor, fuel filter, etc. I would suspect fuel injectors by your description though, you can ohm test the injectors with a voltmeter and see if they are the specified range.

Quick Reply Quick Reply

Register Now

Please enter the name by which you would like to log-in and be known on this site.
If you do not want to register, fill this field only and the name will be used as user name for your post.

Please enter a password for your user account. Note that passwords are case-sensitive.

Please enter a valid email address for yourself.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Bookmarks

Cadillac Posting Rules

  • You may post new threads
  • You may post replies
  • You may not post attachments
  • You may not edit your posts
Read about Lincoln | Buick | Kia Forte Forum
Need products for your Cadillac? Check out your options at the links below:

custom floor mats | Cadillac Chrome and Black Chrome Wheels | window tinting