Cylinder compression numbers for the SD22

[Old Smokey]

Man am I glad to find you guys!

This is my first post about an '82 Datsun 720 diesel that I bought yesterday. I wanted to do a compression test before buying but...spontaneity got the best of me.

I'm still curious though and, having no factory manual, don't know the numbers to look for when doing the test.

Could someone clue me in as to what good cylinder compression numbers are for the SD-22?

Thanks

Andy

[asavage]

Hello, and Welcome!

Compression testing the SD, and compression specifications from the FSM are covered extensively in this post.

You must have a Factory Service Manual (FSM) to work on these critters. The Digital Pump Control (DPC module) and Glow Plug (GP) systems are too weird to diagnose without the FSM. I have the entire Electrical System pages from the FSM online, but you will want to acquire a FSM of your own for the rest of the chapters. Cheapest way to get one is on eBay -- I use this canned search query.

[Old Smokey]

Thanks Al. Yep, an FSM is always the first purchase after buying a vehicle. I'm upgrading rigs from an '81 VW caddy diesel. Well, I consider it an upgrade, at least.

One downside is that parts and info were so easy to find for that lil pickup. I had to search far and wide to find a comparable Nissan forum. VWCaddyforum is just massive and loaded with tons of good info. Glad I found this place though.

Off of that, is there a tried and true source or method for procuring parts for the 720 and SD22? There's always ebay...but sometimes you can't wait for Johnny Q. Diesel to part out the junker that won't run. Any tips would be mucho appreciated.

Thanks again

Andy

[Old Smokey]

I just read through the thread you linked to re: compression testing. You mentioned that you didn't allow your engine to warm before performing the test.

Well...

I'm borrowing a diesel comp tester from my brother who owns an older Benz 300TD. The kit comes with a little booklet written by Kent Bergsma, owner of MercedesSource in Bellingham (where kit was purchased), guru of all things Benzian, and all around good guy.

He writes:

"Let your engine completely cool down before doing the initial test. If possible let it sit overnight and test compression before starting it up the next day. Letting the engine sit idle overnight will allow excess oil to drain away from the piston rings, giving you more accurate cold start readings."

This makes some sense to me. It seems that so much of diesel engine health revolves around starting ability. Bergsma writes that another quick and dirty test is to start the engine with no glow. An engine with good compression should start relatively easy w/o using the plugs.

What are your thoughts on this matter?

Cheers

Andy

[asavage]

Hello, Andy: would you edit your Profile (link at the top of every page) and fill in the Location field? It saves a lot of time down the road, believe me.

I'm borrowing a diesel comp tester from my brother who owns an older Benz 300TD. The kit comes with a little booklet written by Kent Bergsma, owner of MercedesSource in Bellingham (where kit was purchased), guru of all things Benzian, and all around good guy.

Bellingham, Wash.? I lived there for a while. Bergsmas are all over the place.

"Let your engine completely cool down before doing the initial test.

While this contradicts pretty much everyone else, I don't discount this advice, and I do generally perform a cold compression test first.

Parts are machined for optimum performance/correct clearance at operating temperature and many do not fit as well when not at that temp, esp. piston fit to the bore, where pistons are cast and/or machined oval in section, and grow closer to circular when up to temp. Piston-to-bore clearance can affect cranking compression, though not as much as some people think.

I'm usually looking for cyl-to-cyl variation on gassers, and to a lesser extent on diesels. If the numbers are relatively even, and I have a starting problem, then low numbers are definitely a concern, as the engine may be worn out (run without air filter or a cracked plenum hose; WVO operation; run hard when not warmed up; overheated).

Bergsma writes that another quick and dirty test is to start the engine with no glow. An engine with good compression should start relatively easy w/o using the plugs.

While this test may work with the MBs he works with, it certainly won't indicate much on the Nissans I've had.

For example, the LD28 as installed in the Maxima is infamous for being hard to start when warm. The OEM GPC will not activate the GPs if the coolant temperature is above 122°F, and it does need help from the GPs at just below that temp, unless the cetane rating is high, and even then they have to be cranked more than other diesels I've owned and worked with.

I suppose you might be able to start a good LD28 without GPs if the ambient temp was somewhere in the 80's and you have a good starter and battery, but I sure wouldn't lay $10 on a bet.

OTOH, my '83 6.2l, when warm, would start by just tapping the key; it's amazing how easily it would light off.