87 cruises beutifully then gets loud

[SandCrawler]

My 87 454 TBI R20 Suburban with 57,000 miles, cruises along just fine at 70 mph then suddenly the engine gets louder/roars as if it downshifted. Still goes but lacks a bit if top end acceleration. And is loud.

Replaced throttle position sensor as well as egr. Even gave up and took it to a pro and of course nothing happened hadn't happened for a couple months after They replaced the tps but just did it again last night.

Any thoughts? Normally when I start it cold, it goes through its 45 second rev and kick down but this day it didn't.....

 

[1987 GMC Jimmy]

My 87 454 TBI R20 Suburban with 57,000 miles, cruises along just fine at 70 mph then suddenly the engine gets louder/roars as if it downshifted. Still goes but lacks a bit if top end acceleration. And is loud.

Replaced throttle position sensor as well as egr. Even gave up and took it to a pro and of course nothing happened hadn't happened for a couple months after They replaced the tps but just did it again last night.

Any thoughts? Normally when I start it cold, it goes through its 45 second rev and kick down but this day it didn't.....

Are you getting any SES lights? Does it have a TH400 in it? I'm wondering if the detent solenoid is somehow engaging and kicking it down into second gear. Is there any slop in your shifter? I had a sloppy shifter in a motor home that I sold, and when I was in third, I gently bumped it going about sixty, and it fell into second and started roaring. Do you have a tach handy that you can carry with you when you drive it?

 

[SandCrawler]

Are you getting any SES lights? Does it have a TH400 in it? I'm wondering if the detent solenoid is somehow engaging and kicking it down into second gear. Is there any slop in your shifter? I had a sloppy shifter in a motor home that I sold, and when I was in third, I gently bumped it going about sixty, and it fell into second and started roaring. Do you have a tach handy that you can carry with you when you drive it?

No SES lights...yes it does have a TH400...havent heard this theory but could be...when it does this it feels like it looses a bit of power I would have thought if it was in a lower gear it would feel more responsive or "snappy" at the throttle.

 

[89Suburban]

Is it the clutch fan kicking in?

 

[SandCrawler]

Is it the clutch fan kicking in?

Not sure ??? Just drove it again and is louder all the time.. when in drive, if I shift down to 2 it mimics the sound.... so it seems to not be shifting into its top gear?

 

[SandCrawler]

Not shifting at all

Pretty much feels like its iin one gear all the time... gonna check that detent solenoid....

 

[SandCrawler]

Might just be the fan clutch

Not sure ??? Just drove it again and is louder all the time.. when in drive, if I shift down to 2 it mimics the sound.... so it seems to not be shifting into its top gear?

Replacing the fan clutch... was definitly bad and making the "roar", we'll seen if thats the cause of the power drop/acceleration!

 

[glockholiday]

Another vote for the fan clutch here.

 

[SandCrawler]

Another vote for the fan clutch here.

So far so good, smooth as 1987 silk... will update after some more miles... trying to get the 60,000 mile bugs worked out before the MN snow flies.

 

[Rusty Nail]

Fan clutch.

HAYDEN brand only.