Correct cylinder compression PSI?
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Laughing Gas
Joined: Apr 2013
Posts: 1,092
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Correct cylinder compression PSI?
Folks, for a ‘93 351w, what is the correct cylinder pressure? If I remember its about 130-135, but I could be way off. I have some power issues to track down, and this may be apart of it
ill be doing it later this week, and doing some exhaust work because im 99% sure the cat is plugged (stock, 171k on it).
ill be doing it later this week, and doing some exhaust work because im 99% sure the cat is plugged (stock, 171k on it).
Lead Driver
Joined: Jul 2017
Posts: 9,591
Likes: 1,171
From: San Jose, CA
Yep, all 8 plugs out, throttle held wide open, test.
How does it run otherwise? You sure the engine is at fault? Mine's a dog, but I'm pretty sure it's the tall gearing in the differential. It's somewhere down near the 3.00 : 1 range, and I think it would really wake the truck up with 3.55 or lower gearing.
Likely feel like a new truck!
Paul
How does it run otherwise? You sure the engine is at fault? Mine's a dog, but I'm pretty sure it's the tall gearing in the differential. It's somewhere down near the 3.00 : 1 range, and I think it would really wake the truck up with 3.55 or lower gearing.
Likely feel like a new truck!
Paul
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Laughing Gas
Joined: Apr 2013
Posts: 1,092
Likes: 34
Yep, all 8 plugs out, throttle held wide open, test.
How does it run otherwise? You sure the engine is at fault? Mine's a dog, but I'm pretty sure it's the tall gearing in the differential. It's somewhere down near the 3.00 : 1 range, and I think it would really wake the truck up with 3.55 or lower gearing.
Likely feel like a new truck!
Paul
How does it run otherwise? You sure the engine is at fault? Mine's a dog, but I'm pretty sure it's the tall gearing in the differential. It's somewhere down near the 3.00 : 1 range, and I think it would really wake the truck up with 3.55 or lower gearing.
Likely feel like a new truck!
Paul
The motor runs okay. Ive had some intermittent issues with idle variance, my exhaust leak getting worse, and the truck seems to shake more than it used to. I haven’t tracked down the shake, but myself and about 3 shops can’t find a reason for it.
Im 99% sure my CAT is plugged, and since I don’t live in an emissions area..... Im not positive the motor is, but Im also doing this to see the overall state of the motor too.
ive got gremlins to chase down but ive put so much money in around the motor, im starting to believe its not an age issue.
Lead Driver
Joined: Jul 2017
Posts: 9,591
Likes: 1,171
From: San Jose, CA
But not at sea level now? How high is your area? Ran fine when you first got there, or you noticed at least some loss right away?
I've only had one clogged cat personally, but it let me know in no uncertain terms. The engine still ran like a top but kept losing power at lower and lower rpm levels for about a week until it just would hardly get out of it's own way.
Replaced it and back to normal.
Paul
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Laughing Gas
Joined: Apr 2013
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Well gearing sure ain't no problem of yours then! Nice, and wish mine had the low gearing available like my other trucks do.
But not at sea level now? How high is your area? Ran fine when you first got there, or you noticed at least some loss right away?
Where is this exhaust leak? If it's within a foot of an O2 sensor, I'd fix it immediately just because. You never know, but you can guarantee that if it's even remotely close to an O2 sensor, on either side, it's having at least some effect.
Easy to swap out for a straight pipe then. I happen to like cats, and have even put them on older rigs when converting to EFI. But they do get clogged for sure.
I've only had one clogged cat personally, but it let me know in no uncertain terms. The engine still ran like a top but kept losing power at lower and lower rpm levels for about a week until it just would hardly get out of it's own way.
Replaced it and back to normal.
Paul
But not at sea level now? How high is your area? Ran fine when you first got there, or you noticed at least some loss right away?
Where is this exhaust leak? If it's within a foot of an O2 sensor, I'd fix it immediately just because. You never know, but you can guarantee that if it's even remotely close to an O2 sensor, on either side, it's having at least some effect.
Easy to swap out for a straight pipe then. I happen to like cats, and have even put them on older rigs when converting to EFI. But they do get clogged for sure.
I've only had one clogged cat personally, but it let me know in no uncertain terms. The engine still ran like a top but kept losing power at lower and lower rpm levels for about a week until it just would hardly get out of it's own way.
Replaced it and back to normal.
Paul
otherwise ran like a top.
The leak is at the manifold, its on my to do list when I have both time and money. Its gotten worse too.
Power wise, four years ago I could smoke tires in 1st if I tried, used 2nd to start. Now using 2nd feels like a slug, and the truck shudders like I suck at driving stick, even if I rev high and dump it.
I got ahold of the service records, timing chain was replaced at 137k nov 2014. With water pump and a bunch of other goodies.
Based on power loss, an exhaust leak thats getting worse, and a couple other symptoms; im assuming the cat is clogged and creating excessive backpressure making the motor struggle to breathe.
could be other factors in that too, without a doubt but
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Cargo Master
Joined: Jul 2009
Posts: 2,538
Likes: 54
From: Des Moines
A clogged cat is easily tested for using a vacuum gauge. Less than 15” at idle with a fluctuating needle means a restriction.
The next step is to remove the upstream 02 sensor. If vacuum improves, the cat is on its way out.
I’m not saying don’t do the compression test, but if you’re suspecting a restriction, a vac gauge is faster and easier.
The next step is to remove the upstream 02 sensor. If vacuum improves, the cat is on its way out.
I’m not saying don’t do the compression test, but if you’re suspecting a restriction, a vac gauge is faster and easier.
Cargo Master
Joined: Mar 2010
Posts: 3,328
Likes: 292
From: Laramie, WY
Keep in mind cranking compression is going to drop with rising altitude due to a drop in air density. Approximately 3% per thousand feet. So up at 10,000' you're down about 30% compression, and power. This is why turbo diesels and turbo gas engines are so popular (effective) in the high altitudes of the west.
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Laughing Gas
Joined: Apr 2013
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A clogged cat is easily tested for using a vacuum gauge. Less than 15” at idle with a fluctuating needle means a restriction.
The next step is to remove the upstream 02 sensor. If vacuum improves, the cat is on its way out.
I’m not saying don’t do the compression test, but if you’re suspecting a restriction, a vac gauge is faster and easier.
The next step is to remove the upstream 02 sensor. If vacuum improves, the cat is on its way out.
I’m not saying don’t do the compression test, but if you’re suspecting a restriction, a vac gauge is faster and easier.
is there an upstream O2 on a ‘93? I know about the one on the exhaust and I have replaced that.
I will have to see what a vacuum tester will run me. Its rare in my trade to see or use one. Given mechanical is an optional task.
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Laughing Gas
Joined: Apr 2013
Posts: 1,092
Likes: 34
Keep in mind cranking compression is going to drop with rising altitude due to a drop in air density. Approximately 3% per thousand feet. So up at 10,000' you're down about 30% compression, and power. This is why turbo diesels and turbo gas engines are so popular (effective) in the high altitudes of the west.
If I had the money right now I would add a turbo with a computer upgrade and some other goodies. Thats a long ways out though. One day I hope a turbo and a 408 stroker kit is in the motors future.
Cargo Master
Joined: Jul 2009
Posts: 2,538
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From: Des Moines
Vac gauges are usually less than $50 off the shelf - I’m sure you can get it cheaper online.
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Laughing Gas
Joined: Apr 2013
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Ill have to look for that on Friday and see
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Laughing Gas
Joined: Apr 2013
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Likes: 34
Well, after two hours of griping, the cat isn’t coming out.... itll have to be cut out.
So, I suppose this summer a complete exhaust redo is in order.
ill get the compression test done soon enough.
So, I suppose this summer a complete exhaust redo is in order.
ill get the compression test done soon enough.
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