How Low Can You Go? - Compression (1 Viewer)

[fuzzywuzzy]

I figure I must be doing something wrong.... I just purchased a 91 with the 3FE motor. When I went to have it smogged prior to transferring title it failed. I went to do a minor tune-up (plugs, wires, cap, and rotor) and figured while I was at it I'd go ahead and check the compression. While the vehicle did fail smog it does run strong with some minor valve chatter. I borrowed the compression tester from my local O'Reilly's. Here's my process and I'm sure it's not correct...

• Engine was cold (I know it should be running temp but I was already doing the tune-up.. some sites suggested that there's normally about a 10% drop in pressure when checking hot vs. cold)
• All plug wires were off, only cylinder 1 plug was removed (later #2) but nothing else was removed or loosened.
• Screwed in air hose, connected gauge, cranked about 6 times.
• Cylinder #1 read 50 psi and when I installed a new plug for #1 and checked #2 it read 60 lbs.
• Did not add oil to check rings vs. valves

Is compression this low even possible on a vehicle that seems to be running seemingly strong? I'm thinking there's either something wrong with my method or the gauge.

 

[RockDoc]

Remove all spark plugs, crank it over with the throttle wide open. I'd pull the EFI fuse as well so it won't deliver fuel.

 

[flintknapper]

Remove all spark plugs, crank it over with the throttle wide open. I'd pull the EFI fuse as well so it won't deliver fuel.

^^^^^^^^^ This.

As RockDoc has said.


Additionally....IF you have a battery charger, hook it up and put it on the highest setting (up to 75 amps)...so that when you crank the engine you have the most consistent battery power possible. Often times, folks have a marginal battery and by the time they get to cylinder #6 the battery isn't turning the starter as hard as with the previous cylinders, resulting in a lower pressure reading for that cylinder.

 

[IanB]

That IS low. Hopefully the results when you re-test are better.

-warm engine
-EFI fuse removed so no fuel is injected
-all sparkplugs removed so the engine spins easier
-battery charger so you get consistant rpm throughout testing
-throttle fully open when cranking
-make sure tester is fully tightened so air isn't able to escape past the threads

The actual #'s can vary for a number of reasons, what I look for is all cylinders being within 10% of each other. 50 and 60psi is quite low though, I'd consider testing on another vehicle to ensure the gauge is even in the right ballpark, being a loaner unit it may have had a hard life.

 

[fuzzywuzzy]

That IS low.

As in not possible low, likely user error?

-throttle fully open when cranking

by having foot on gas to the floor, right? nothing to be done in engine bay?

I'd consider testing on another vehicle to ensure the gauge is even in the right ballpark, being a loaner unit it may have had a hard life.

good idea and agreed. It's only about $25-30 for a new bosch unit so might just buy one.

 

[IanB]

As in not possible low, likely user error?

by having foot on gas to the floor, right? nothing to be done in engine bay?

good idea and agreed. It's only about $25-30 for a new bosch unit so might just buy one.

I'm hoping user error, or a totally out to lunch gauge for your sake. I'd question whether it would even run being that low!

Foot on the gas when cranking, you want the throttle butterfly open so there's no restrictions in the intake limiting the air the cylinder can take in when cranking.

For $30, I'd splurge for a new tester.

 

[Rivman1243]

Don't forget to ground out the coil wire to the frame.

 

[fuzzywuzzy]

Don't forget to ground out the coil wire to the frame.

What does this help with?

 

[vipergrhd]

^^^ So you won't get shocked and saves the coil. Like what others have said warm engine and remove all plugs. Crack open the throttle and crank it until the needle stops moving get 2 readings from each cylinder and them do a wet compression test. Any cylinder with 70 psi or below will no longer ignite the mixture and would cause a bad misfire.

 

[fuzzywuzzy]

Wet test all cylinders? Figuring why not considering all will cylinders will have plugs removed at that point. Will having oil in every cylinder cause any starting issues when everything is buttoned back up?

So this is the plan for this weekend:

• Remove all plugs, remove EFI fuse, ground coil
• pray
• do compression test on all cylinders 2x each dry
• pray
• do wet test 2x each

Button everything back up.

 

[IanB]

A capful of oil in the cylinder won't cause any issues with starting. The cranking for the wet test will pretty well smear and spread it around the cylinder walls and into the rings anyways. Good luck!

 

[flintknapper]

What does this help with?

Avoids ruining your Henway.

 

[dugsgms]

Post a pic of the failed smog report, maybe we can help to get you to pass.

 

[LINUS]

X2 on using a different guage- those loaners are shown zero respect.

All the tips above, and a critical eye to the threads of the lower section, and making sure the connector (if the 2-piece style, all I see anymore) seats correctly.

 

[fuzzywuzzy]

Here ya go @dugsgms - 2016 test posted first where it passed, my most recent test is below, failed on hydrocarbons - i.e. unburnt fuel. A little internet checking suggests it just might not have been hot enough. The cap/rotor/wires are new. The plugs were new as well and looked to be gapped correctly but they were the super-nice iridium plugs. I put OE plugs in and seems to be running a little smoother but there is a little valve chatter. Lastly, the tech on my test from last week suggested checking timing, said it looked ok but couldn't get good reading and may have been a little off but he passed it for timing.

 

[vipergrhd]

Check your PCV valve and hoses make sure its all flowing, changed the oil and add some Marvel Mystery oil. Run a bottle of Seafoam in your fuel system and it should drop that HC in a heartbeat. You did not post what is your current mileage but if your pushing 7 years and near 100k since the last TB/WP job I would do it before that long trip. Not trying to scare you one of my cars blew a cam seal in my drive way one evening while I was checking all my fluid level before the start of the work week and I'm so glad it happened then coz if it did happened on the road it will be too late by the time the oil light comes on. Just my 2 pennies.

 

[vipergrhd]

Pardon the last part of my posting about the cam seal it was meant for another guy here.
I know you don't have a timing belt.

 

[fuzzywuzzy]

Currently have Seafoam running through now, making it my daily driver for the remainder of the week to get it burned out and refill with fresh fuel. I think I'll have the time to do the oil change and add the marvel as well. And you had me freaked out with the WP for a second!

 

[dugsgms]

Okay, ran the numbers through a Lambda calculator. Good news is that engine is in pretty good fuel control, bad news is that catalytic convertor(s) are barely working. On the old test cat was working at about 74% efficiency, new test was 33% on the idle portion and and a paltry 3% on the 2500rpm. Assuming you don't have any misfiring(rough running or check engine light along with p0301-p0306 dtcs) its a fairly safe bet that you cat(s) are shot.

You may be able to barley squeak by by super heating the cats(revving the engine up to 2000-3000rpms for a few minutes right before the tailpipe portion of the test) but even then its gonna be close.

 

[fuzzywuzzy]

Very good info, thankfully not a very expensive part to replace. Not to seem like a dick but you're fairly confident? If so I'm thinking I'll cancel my appointment with the smog check place on Friday and order this cat (Magnaflow Catalytic Converters - 50 State California Legal).
Do the oil change recommended by @vipergrhd and register the rig as NOP for the week or two until the cat comes in and I replace it along with the pcv valve (assuming it's necessary). Also considering a drop-in replacement K&N replacement air filter.