Compression Testing Saga - PeachParts Mercedes-Benz Forum

mach4 · #1 09-02-2012, 08:50 PM

I've been working through a problem with a rough idle that has just recently become what feels like a gasser running with one or two spark plugs unplugged.

The first step was to pop test all the injectors and see if I could find a suspect injector. The results on my new DIY tester were as follows

1- 1850# good spray
2- 1800# good spray
3- 1800# good spray
4- 1850# good spray
5- 1800# generally good spray, but streams

I replaced #5 with a 1850# injector with a good spray from a junk yard set I had.

It still ran the same.

So I decided to do a compression check and this is where it got interesting. I picked up a Harbor Freight unit and then proceeded to pull out all the injectors. I had been reading up on doing compression tests and the jury was out on whether to use the heat shields or not, so I decided to use them. I screwed the test adapter into the #1 injector slot. It kind of had some resistance through a few pulls then snugged up nicely. I cranked the engine and it registered 410#. Not a bad sign.

But when I pulled out the test fitting, this is what I found

The injector test fitting had literally punched the center out of the heat shield. It didn't register what happened at the time, so I went on to #2, #3, #4 and #5. Then it hit, where is the center piece of the heat shield. Panic time. Amid visions of replacing the pre chamber, pulling the head and possibly damaging a valve or worse, I found the center piece had been blown out on the ground.

So here's what we've got. Notice how the bottom of the injector is different from the compression test fitting in the HF kit?

There are actually three problems. The first is with the configuration itself and getting a good seal. But there is another problem and that is the mismatch in the size of the bottom piece. The arrows show how much more volume there is under compression compared to with the injector in the cylinder. Some threads have speculated that the hole in the adapter will affect the compression ratio, but this is dramatically more than that.

And the last is the quality of the kit itself. The first one would not release the pressure (I took it back) and the second one would not hold pressure (I'm going to take that one back too)

Anyway, here's my results on a cold engine

1- 350#
2- 360#
3- 350#
4- 310#
5- 310#

Notice that the above results reflect redoing #1. The first test was 410# and the next one was #350. I can only conclude that the mismatch in the injector-tester configuration produced the difference. With the heat shield "jammed" in and eliminating the mismatch area, it read considerably higher.

I was going to cross reference to readings at the glow plugs but gave up when I realized that 2, 4 and 5 were inaccessible.

I believe it is possible to fabricate a good test adapter from an injector and some JB weld. The connection to the compression tester is a simple 1/8"NPT fitting. I'll post any success in creating a good test fitting.

After everything is back together, I've still got a rough idle when cold, but the missing and hot idle roughness have gone away.

Diesel911 · #2 09-02-2012, 10:31 PM

Quote:

Originally Posted by mach4

I've been working through a problem with a rough idle that has just recently become what feels like a gasser running with one or two spark plugs unplugged.

The first step was to pop test all the injectors and see if I could find a suspect injector. The results on my new DIY tester were as follows

1- 1850# good spray
2- 1800# good spray
3- 1800# good spray
4- 1850# good spray
5- 1800# generally good spray, but streams

I replaced #5 with a 1850# injector with a good spray from a junk yard set I had.

It still ran the same.

So I decided to do a compression check and this is where it got interesting. I picked up a Harbor Freight unit and then proceeded to pull out all the injectors. I had been reading up on doing compression tests and the jury was out on whether to use the heat shields or not, so I decided to use them. I screwed the test adapter into the #1 injector slot. It kind of had some resistance through a few pulls then snugged up nicely. I cranked the engine and it registered 410#. Not a bad sign.

But when I pulled out the test fitting, this is what I found

The injector test fitting had literally punched the center out of the heat shield. It didn't register what happened at the time, so I went on to #2, #3, #4 and #5. Then it hit, where is the center piece of the heat shield. Panic time. Amid visions of replacing the pre chamber, pulling the head and possibly damaging a valve or worse, I found the center piece had been blown out on the ground.

So here's what we've got. Notice how the bottom of the injector is different from the compression test fitting in the HF kit?

There are actually three problems. The first is with the configuration itself and getting a good seal. But there is another problem and that is the mismatch in the size of the bottom piece. The arrows show how much more volume there is under compression compared to with the injector in the cylinder. Some threads have speculated that the hole in the adapter will affect the compression ratio, but this is dramatically more than that.

And the last is the quality of the kit itself. The first one would not release the pressure (I took it back) and the second one would not hold pressure (I'm going to take that one back too)

Anyway, here's my results on a cold engine

1- 350#
2- 360#
3- 350#
4- 310#
5- 310#

Notice that the above results reflect redoing #1. The first test was 410# and the next one was #350. I can only conclude that the mismatch in the injector-tester configuration produced the difference. With the heat shield "jammed" in and eliminating the mismatch area, it read considerably higher.

I was going to cross reference to readings at the glow plugs but gave up when I realized that 2, 4 and 5 were inaccessible.

I believe it is possible to fabricate a good test adapter from an injector and some JB weld. The connection to the compression tester is a simple 1/8"NPT fitting. I'll post any success in creating a good test fitting.

After everything is back together, I've still got a rough idle when cold, but the missing and hot idle roughness have gone away.

My Harbor Freight Compression Test Kit has an adapter, a Tube/Ring and a couple of Copper Washers. I had always assumed some combination of them would work.

mach4 · #3 09-02-2012, 10:46 PM

Quote:

Originally Posted by Diesel911

My Harbor Freight Compression Test Kit has an adapter, a Tube/Ring and a couple of Copper Washers. I had always assumed some combination of them would work.

Good call. I just took a look and you're right. It appears that the adapter, then the copper washer, then the spacer and finally a heat shield would work perfectly. I'm not going to rip it apart to retest right now, but that's the answer.

I'm still dissatisfied with the overall quality of the kit. Maybe #3 is the charm.

Thanks

Diesel911 · #4 09-02-2012, 11:30 PM

Quote:

Originally Posted by mach4

Good call. I just took a look and you're right. It appears that the adapter, then the copper washer, then the spacer and finally a heat shield would work perfectly. I'm not going to rip it apart to retest right now, but that's the answer.

I'm still dissatisfied with the overall quality of the kit. Maybe #3 is the charm.

Thanks

My whole kit I bought for $27 when it was on sale.
Go take a look on eBay and see what just one if the Injector Adaptors will cost.
Cheap Tools are what they are; sometimes they don't work well or are a pain to use. Overall the stuff I have bought from Harbor Freight has at least been adequate for the limted uses I put them through.

Diesel911 · #5 09-02-2012, 11:33 PM

Way back I converted an Gasser Compression tester setup to diesel by switching the Gauge.
It worked OK but the setup used Tire Valve Stem Schrader Valves as the valve in it. I found that the heat of compression developed by a Diesel can burn up the seal in the Schrader Valve.
I suspect that the Harbor Freight Tools also use the Schrader Valves.

t walgamuth · #6 09-03-2012, 06:11 AM

I believe my tester may be from harbor freight. I think my Indie took an old injector and made an adapter from it. Your compression results suggest you are getting a good seal now.

The engine is supposed to have a device which steps up the idle a little when cold. My 300/240 conversion lacks this and idles too slowly. This will cause rough idle for sure and it will be less so when warm.

I just use my foot for the fast idle til warm. With a stick that is pretty simple.

WINGAS · #7 09-03-2012, 09:11 AM

I had to go with a higher end piece from Napa, with an adapter for my 606 style injectors. $90 versus $25 sucks, but not being all that good at this stuff, I am required to eliminate anomolies due to shoddy equipment.

funola · #8 09-03-2012, 09:54 AM

you didn't read the directions? which as I recall tells you which adapter and washers to use for a particular engine.

the hf kit has too many fittings that can leak to be a good reliable compression tester. I modified mine extensively and it is finally reliable, quick and easy to use.

Quote:

Originally Posted by mach4

I've been working through a problem with a rough idle that has just recently become what feels like a gasser running with one or two spark plugs unplugged.

The first step was to pop test all the injectors and see if I could find a suspect injector. The results on my new DIY tester were as follows

1- 1850# good spray
2- 1800# good spray
3- 1800# good spray
4- 1850# good spray
5- 1800# generally good spray, but streams

I replaced #5 with a 1850# injector with a good spray from a junk yard set I had.

It still ran the same.

So I decided to do a compression check and this is where it got interesting. I picked up a Harbor Freight unit and then proceeded to pull out all the injectors. I had been reading up on doing compression tests and the jury was out on whether to use the heat shields or not, so I decided to use them. I screwed the test adapter into the #1 injector slot. It kind of had some resistance through a few pulls then snugged up nicely. I cranked the engine and it registered 410#. Not a bad sign.

But when I pulled out the test fitting, this is what I found

The injector test fitting had literally punched the center out of the heat shield. It didn't register what happened at the time, so I went on to #2, #3, #4 and #5. Then it hit, where is the center piece of the heat shield. Panic time. Amid visions of replacing the pre chamber, pulling the head and possibly damaging a valve or worse, I found the center piece had been blown out on the ground.

So here's what we've got. Notice how the bottom of the injector is different from the compression test fitting in the HF kit?

There are actually three problems. The first is with the configuration itself and getting a good seal. But there is another problem and that is the mismatch in the size of the bottom piece. The arrows show how much more volume there is under compression compared to with the injector in the cylinder. Some threads have speculated that the hole in the adapter will affect the compression ratio, but this is dramatically more than that.

And the last is the quality of the kit itself. The first one would not release the pressure (I took it back) and the second one would not hold pressure (I'm going to take that one back too)

Anyway, here's my results on a cold engine

1- 350#
2- 360#
3- 350#
4- 310#
5- 310#

Notice that the above results reflect redoing #1. The first test was 410# and the next one was #350. I can only conclude that the mismatch in the injector-tester configuration produced the difference. With the heat shield "jammed" in and eliminating the mismatch area, it read considerably higher.

I was going to cross reference to readings at the glow plugs but gave up when I realized that 2, 4 and 5 were inaccessible.

I believe it is possible to fabricate a good test adapter from an injector and some JB weld. The connection to the compression tester is a simple 1/8"NPT fitting. I'll post any success in creating a good test fitting.

After everything is back together, I've still got a rough idle when cold, but the missing and hot idle roughness have gone away.

barry123400 · #9 09-03-2012, 01:08 PM

Quick test. Shut off the return line from the injection pump and see if the idle is better. Indicated compression readings are not so far off that the idle should be noticeably bad from that anyways. If you have not checked for valve lash do so. I am assuming this is an older 617 engine that has mechanical lifters.

Diesel911 · #10 09-03-2012, 02:54 PM

Quote:

Originally Posted by barry123400

Quick test. Shut off the return line from the injection pump and see if the idle is better. Indicated compression readings are not so far off that the idle should be noticeably bad from that anyways. If you have not checked for valve lash do so. I am assuming this is an older 617 engine that has mechanical lifters.

The IP Return line is hard Plastic; there would be more luck with the Cigar Hose. If the Cigar Hose is old it will also be too hard and stiff just to pinch off.

He would be better off temporarily replacing the Cigar Hose with a section of 5/16" Fuel Hose; that can easily be pinched off with a Pliers or a Vice Grips.

If the idle gets better by pinching off the return Hose you have an issue with your Pressure Relief/Overflow Valve.

mach4 · #11 09-03-2012, 03:16 PM

Quote:

Originally Posted by Diesel911

He would be better off temporarily replacing the Cigar Hose with a section of 5/16" Fuel Hose; that can easily be pinched off with a Pliers or a Vice Grips.

If the idle gets better by pinching off the return Hose you have an issue with your Pressure Relief/Overflow Valve.

I don't have a cigar hose on the engine - just 5/16 fuel hose. Just tried clamping the return line off and not change.

Stretch · #12 09-04-2012, 02:32 AM

It seems as though most people ignore the glow plug hole as a connection for a compression tester / leak down tester - is that right?

Govert · #13 09-04-2012, 04:20 AM

Quote:

Originally Posted by Army

It seems as though most people ignore the glow plug hole as a connection for a compression tester / leak down tester - is that right?

The IP can be in the way, depending on the connectors of the compression tester and the position of the IP.

t walgamuth · #14 09-04-2012, 06:15 AM

Quote:

Originally Posted by Army

It seems as though most people ignore the glow plug hole as a connection for a compression tester / leak down tester - is that right?

I'd say no, some use it. I like the injector hole but either should work fine depending on what fittings you have. I think the injectors are much easier to get to.

JHZR2 · #15 09-04-2012, 08:31 AM

Nice report, but remember - youre buying China Freight and being dissatisfied? All they are is a job stealing seller of inferior parts that should be made here.

I personally use a tester that goes through the GP hole, since removing/checking/reaming should be a PM job when checking any of this stuff anyhow.