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Author Topic: Replacing spark plugs on a 90 Deville 4.5  (Read 749 times)

Offline TJ Hopland

  • Posts: 10183
Replacing spark plugs on a 90 Deville 4.5
« on: December 28, 2018, 11:13:37 AM »
Just finished this project.  I don't know that its the worst engine I have ever done plugs in but its got to be close.  There are some vans I have worked on that were worse.  At least its iron heads so you don't have too big a concern about stripping and seizing.   If you are the type of person that likes or has to be able to see what you are working on forget about this project, there are only 2 plugs you can sort of see and you for sure can't see em and touch em at the same time.   I'm assuming the process would be similar on any of the transverse 4100, 4.5, or 4.9 engines.   

I used a standard 3/8 drive spark plug socket and on most a 3" extension.  Ratchet I used was a fairly long handle swivel head with a fairly fine ratchet.  On 6 I used a 1" extension.   I also used what amounts to a ratchet with no handle to help turn them out once I got them loose since there just isn't room to swing a normalish ratchet more than one click at a time.   Without that swivel head that I recently bought for another project on this car I think I would still be out there swearing at the thing 20 hours later.   That may be my new go to ratchet.  I think the whole project was about 2 1/2 hours and I am very sore today from laying on the engine and all the odd angles.

For the back/even side I removed the plastic cowl cover that covers up some wiring harness stuff.  That gives you a little extra room to work.  2 and 4 are not terrible and you could maybe reach those from below.  6 isn't great because there is an AIR tube and the O2 sensor making difficult access more difficult.  8 is pretty bad because you are still near that AIR tube and some other sensor plus you are next to the trans dipstick tube.   I would say that its a tie between 8 and 3 for being the worst ones and the ones that take the longest.  6 you can't see or touch from below, its too close to the exhaust downpipe.   8 you can see from below but I didn't think the access was any better from below than above.

For the front/ odd side which is usually not that bad on transverse engines there is only #7 that is maybe be considered easy to get to and even that wasn't that easy.   5 is a little harder but not terrible.  To try and make 1 and 3 easier I pulled the AIR tubes off the plenum and diverter assembly then pushed the hoses on the pump out of the way to make a little more room.  #1 was a pain but was dooable.   #3 was pretty bad.  There is just no clearance with the radiator fan and its down where the power steering bracket splits.   Like I said 3 and 8 were the fun ones that I would say I spent close to 45 mins on each.   None of these looked accessible from below.   I forgot to look and see how hard it would be to remove the fan for a little more clearance.           
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Offline harry s

  • Harry Scott 4195
  • Posts: 1920
Re: Replacing spark plugs on a 90 Deville 4.5
« Reply #1 on: December 28, 2018, 11:41:33 AM »
You brought back some memories of changing plugs in a 4.5. I remember loosening the subframe for the back plugs. Northstars are no picnic either.     Harry
Harry Scott 4195
1941 6733
1948 6267X
2011 DTS Platinum

Offline WTL

  • Posts: 251
  • Name: W. Love
Re: Replacing spark plugs on a 90 Deville 4.5
« Reply #2 on: December 28, 2018, 01:25:47 PM »
Makes me wonder if the dealerships had a special tool/procedure. 

Offline gkhashem

  • Posts: 1117
  • CLC Number: 26576
  • Name: George K Hashem
Re: Replacing spark plugs on a 90 Deville 4.5
« Reply #3 on: December 28, 2018, 01:29:34 PM »
As I remember now after the comments, when I had someone do my fuel injectors and the plugs. He did mention it was a tough job to get them in.
1959 Oldsmobile 98 Holiday Sports Sedan
1964 Oldsmobile 98 Town Sedan (OCA 1st)
1970 GMC C1500
1977 Oldsmobile 98 Regency Coupe
1978 Cadillac Coupe Deville (Sr Crown #959)*
1984 Oldsmobile Delta 88 Royal Brgham Coupe (OCA 1st)
1991 Oldsmobile Toronado Trofeo (OCA 1st)
1991 Cadillac Eldorado Biarritz (Sr #838)
1992 Oldsmobile 98
1996 Oldsmobile 98

Offline TJ Hopland

  • Posts: 10183
Re: Replacing spark plugs on a 90 Deville 4.5
« Reply #4 on: December 28, 2018, 03:06:47 PM »
I can't quite imagine what sort of special tools you could use other than maybe some fairly petite acrobat kind of a person with small but strong hands that doesn't mind being in awkward positions for extended periods of time.  I have more tools than the average person plus a oxy torch, press, lathe, mill so if it was a matter of something with a different bend or angle on it I would have done it.   I'm pretty sure engines and setups like this are why they came up with the 100k tune ups which bring in their own set of problems.   Not only is the stuff darn near impossible to access its now been corroding in there for 100k so even if you can get to it its not going to give up without a fight.

It would only be slightly easier if you removed a bunch of hoses and other assemblies.  Back when I had a 95 with a 4.9 I think the main issue was a bad fuel pressure regulator.   My friend that helped said as long as we were in there we should do the plugs, wires, valve cover gaskets, and a few other things.   I don't remember the plugs being the worst of that so having all that stuff off must have made it a little easier.  I remember that being an all day project with 2 people including one that had done it before.

If you pulled the fans and radiator which may involve disconnecting an AC line that is in the way that would make the front much easier.   With that stuff out of the way then maybe you could loosen some mounts and roll the whole thing forward to make some room in the back.

Most annoying thing about the project is it didn't cure my under load vibration problem.   I was excited when I found a couple of the plugs loose but that didn't seem to be hurting anything.   I figured I would find one cruddy one from running with bad injectors or bad computer or one that someone just gave up on and didn't change but they all looked good.  They were AC but had smooth insulators and a different number, maybe recent production or a different quality line?   The ones I put in were normal looking and the part number out of the manual.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Offline Barry M Wheeler #2189

  • Administrator
  • Posts: 3489
  • CLC Number: 2189
  • Name: Barry Wheeler
Re: Replacing spark plugs on a 90 Deville 4.5
« Reply #5 on: December 28, 2018, 04:44:10 PM »
On my 1991 Seville (4.9), my vibration has mostly gone away. At least the wiggling from 55-60 MPH has pretty well smoothed out. I still get some RPM variation at idle that has settled down to a total of fifty when the engine is warmed up. (625-675) 625-650-675-650-625 and so on.

Mileage has settled down to an average of 20.3. Round trip to town is 50 miles, so it's getting warmed up nicely at least once a week.

Total mileage is presently 22,975. When I got it last year, my mechanic put 5W-30 oil in it and I've been getting some valve clatter at start-up.  I had it changed yesterday and had them put 10W-30 in it and the short distance I've driven it around town seems to have quieted down some with the heavier mix.
Barry M. Wheeler #2189

1981 Cadillac Seville
1991 Cadillac Seville

Offline Dave Shepherd

  • Posts: 2422
Re: Replacing spark plugs on a 90 Deville 4.5
« Reply #6 on: December 28, 2018, 07:17:05 PM »
That engine was never designed for 5/30 unless driven in constant temps below freezing.


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