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425 engine tune up


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Hello evrybody!

I drive a 1978 cadillac coupe deville...and i've run into a little problem...

How do I...or what is the best/easiest way to change the spark plugs. specifically the last one on the left side(grill p.o.v.)

looks impossible...

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have you tried from under the car? I have reached many difficult to get at plugs over the years from under the car

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Interest question...

I did plugs in my Dads 78 Sedan, and don't remember any unusual issues getting to the plugs... other than my arms looked like you had used a file on them... they were so scratched up from the back plug and all the "STUFF" that you had to work your arm around to get to it.

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I have also heard of removing the tire/wheel and having to cut a hole through the wheelwell to get to certain plugs. Not this particular model, but it's a possibility. Some vehicles actually had a little "cut here" outline stamped in the wheelwell, then a small piece of metal was used to cover the hole.

Never underestimate the amount of a persons greed.

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You can access some of the plugs by removing the front tire and lifting the tar paper flap that covers the camber alignment hardware. That may provide additional access to some of the plugs.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I drive a 1978 cadillac coupe deville...and i've run into a little problem...

How do I...or what is the best/easiest way to change the spark plugs. specifically the last one on the left side(grill p.o.v.)

looks impossible...

All the plugs can be reached from above while leaning over the fenders. Removing wheel assemblies is for Chevrolet V8s, where the plugs sit under the exhaust manifolds. :) In the case of #7 (passenger side, rear-most), remove all swivels and extensions, place the sparkplug socket over the plug, then attach your standard 3/8" drive ratchet. When this series of Cadillac V8 is mounted in a longitudinal FWD chassis, the plugs can be changed in less than 10 minutes assuming they're gapped ahead of time; add another couple of minutes for #7 in the RWD chassis. :)

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You can access some of the plugs by removing the front tire and lifting the tar paper flap that covers the camber alignment hardware. That may provide additional access to some of the plugs.

This was my solution for the front passenger side on my 1996 Corvette with the 427 engine. That plug was under the A/C compressor and was a bear to get to from the top. For the fastest plug change, I turned the steering full left, whipped out the scissor jack and propped up the frame a couple of inches, and used three feet of extensions; I sat on a little shop stool to spin that plug off and the new one on. I could change all eight in about 15 minutes that way, with all due care in just the right torque, etc. But, it looks like KevinW's solution is best for the big-block Cadillacs.

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It's been so long since I saw a big block Cadillac engine that I forgot the plug location.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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thanx for all the replies

got a feelin the #7 plug has never been changed...b'cuz of its position :P

did a 0-60 today...n it took forever :(

slower than a 1985 brougham wit a 4.1

That ain't right at all...

My Dad had a 78 Sedan and it would smoke the back tires... :D

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  • 2 weeks later...

finally got all the sparkplugs...n all that changed...

the 0-60 acc. is now at 14sec...n it feels a lot stronger :)

though....it should b faster right?

also....when/at what speed should it shift into second? and third? rpm?

The 1977-79 C-Body Cadillacs with the standard 2.28:1 final drive ratio should be able to hit 60 MPH in just over 10 seconds under ideal conditions. The WOT 1-2 shift should occur close to 55 MPH at around 4000 RPM (peak HP). It is possible to do 0-60 MPH in first gear if your valve-springs are in good shape. :) WOT 2-3 should occur around 80 MPH.

Something to keep in mind if everything isn't original is the OEM transmission torque converter has a relatively high stall speed, to complement the tall gearing; most generic replacement converters do not offer the same performance, but this alone wouldn't account for your performance observation. Have you verified the speedometer calibration with a GPS unit? Another issue with transmission swaps is the speedometer drive and driven gears become mismatched because most other THM400 applications have numerically higher drive ratios.

Remove the distributor cap and rotor, and ensure that the centrifugal advance mechanism moves freely, and inspect for wear around the weight pivots. Adjust the ignition timing to 18 degrees BTDC at 1400 RPM while the vacuum advance is disconnected.

Edited by KevinW

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  • 3 weeks later...

plugs, wires n distributor are all new now...the timing i think was set to 21...22 or 23... was at 4 or somethin before the swap :P

acc 0-60 in 12sec now...shifted into second at 50-55mph

checked the speedometer wit a gps last summer...so thats all good

now i think i gotta get a new generator though...cuz the generator light is on all the time now :(

Edited by Charles Edberg
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i think i gotta get a new generator though...cuz the generator light is on all the time now :(

Take the alternator to a local rebuilder and have it rebuilt vs. putting a chain store POS alternator on it.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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plugs, wires n distributor are all new now...the timing i think was set to 21...22 or 23... was at 4 or somethin before the swap :P

acc 0-60 in 12sec now...shifted into second at 50-55mph

checked the speedometer wit a gps last summer...so thats all good

now i think i gotta get a new generator though...cuz the generator light is on all the time now :(

Regarding the new distributor, many generic vacuum advance units do not match the characteristics of the original unit, so if you observe some part throttle detonation (pinging), you may need to limit the range of movement. There may still be some room for improvement. How's the fuel economy? It should be relatively easy to achieve the EPA ratings of 14/17 MPG US (city/hwy).

BTW, that 63 amp 10SI (or possibly an optional 80 amp unit) is one of the easiest generators to change and rebuild - a great learning experience. Maybe all it needs is new brushes...

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