Do i need to do a compression test before i buy? - PeachParts Mercedes-Benz Forum

ambush276 · #1 07-09-2016, 12:03 AM

I am looking for a 300td wagon to complement my 240d (need a car with more space). Now that i am more educated on this topic i know the proper thing to do is a valve adj then compression test before i make a purchase.

Is there a way to tell if the engine has fine compression without this?

For example, if i start the car on a cold idle, it starts up quick and the idle is fairly smooth... is this an indication of good compression? Or... are there still other gremlins that may hide with bad compression.

I am basically trying to think of reasonable tests so when i go see a car, i don't spend 1-2 hrs do a valve adjst... then compression test.

any suggestions?

Thanks.

NZScott · #2 07-09-2016, 01:27 AM

How long it takes to start without glowing is a good test I find.
It never gets proper cold where I live though

neumann · #3 07-09-2016, 06:24 AM

Quote:

Originally Posted by ambush276

I am looking for a 300td wagon to complement my 240d (need a car with more space). Now that i am more educated on this topic i know the proper thing to do is a valve adj then compression test before i make a purchase.

Is there a way to tell if the engine has fine compression without this?

For example, if i start the car on a cold idle, it starts up quick and the idle is fairly smooth... is this an indication of good compression? Or... are there still other gremlins that may hide with bad compression.

I am basically trying to think of reasonable tests so when i go see a car, i don't spend 1-2 hrs do a valve adjst... then compression test.

any suggestions?

Thanks.

Has anyone ever agreed to let someone do a valve adjustment on their car as part of a prepurchase inspection? Are you having a shop do the valve adjustment? I would never let someone do that unless they can prove that they are certified or I know their skills in advance.

97 SL320 · #4 07-09-2016, 07:05 AM

How deep one digs into a car is dependent on how much the car costs relative to the market.

If someone is asking a premium price, the car gets a through going over. If the car is a "motor runs rough and needs work" I consider the motor bad and deduct the cost of replacement from market value.

I consider old cars with high / suspect miles are considered to have failing engines / transmissions / worn parts, if those systems turn out to be good, that is a bonus. Don't consider old cars a commodity like buying a 4 year old _fill in the blank_ where the chances of buying a good one sight unseen are good.

I've been in and around the auto repair business for over 40 years. If someone is asking for a compression test on a $ 1,000 car I'm selling , I call it a no sale because the low end buyers are typically the biggest whiners as they expect a 100% new car for no money.

If the car was selling for $ 10,000 and was absolute top of market, I'd do a compression test while they were there ( with a non refundable deposit ). For the most part, the higher end buyer is more accepting and realistic on what a used car is and isn't.

There are exceptions on either end of the scale but that is how things generally track.

BillGrissom · #5 07-09-2016, 07:07 AM

A few simple tests should be sufficient. Remove the oil fill cap. If you see excessive blow-by, especially puffing, the rings are worn. See youtube's to judge. If you see little flow from just the smaller PCV tube, the engine is excellent. Valve adjustment won't improve blow-by. Another test is to turn the engine over by hand and feel for each cylinder strongly resisting you (5 per 2 rev's). I use the power steering pulley bolt to turn it. If an owner didn't allow such tests, I would just walk. The world of potential 300D buyers is very small, and 300TD even smaller, so not smart to be fussy with buyers.

leathermang · #6 07-09-2016, 10:24 AM

Quote:

Originally Posted by ambush276

......Is there a way to tell if the engine has fine compression without this?......

No.
You can have great starting and still have one or more cylinders low on compression.

Valve adjustments are supposed to be a regular maintenance procedure .. thus one might assume that a seller would have one done before putting the car up for sale....

So if everything else looks good... I assume you are close enough to one of those ' Lemon Busters' type of mobile and independent ( they do not do any repairs, so no skin in the game ) services....

I would expect a seller to allow the car to be checked by that service...
I think they are about $100-130 for the check which includes many things in addition to compression...

Buyer should offer to pay for it...and the results should belong to them.

t walgamuth · #7 07-09-2016, 10:36 AM

Generally if the warm idle is smooth and there is not excessive blowby the condition of the bottom end is good. Quick start up is also a very good sign.

Jeremy5848 · #8 07-09-2016, 11:29 AM

If you can find a mechanic who works on these cars every day (harder now for the older ones) or at least has worked on them for years, have the pre-purchase inspection done at that shop. They will know what to listen/look/feel for.

ambush276 · #9 07-09-2016, 11:44 AM

thanks for all the tips everyone. I know how to do the valve adjust and compression test, i've done it a couple of times on a 240d and a 300d. But as said... most owners probably will not let me do this check. It seems that if it has a good start... low blow back... and a decently smooth idle i can have some assurance that the car will last for a decent amount of time without worrying about a motor quitting. (obviously timing chain check too!)

MagicBus · #10 07-09-2016, 12:38 PM

Quote:

Originally Posted by 97 SL320

If someone is asking for a compression test on a $ 1,000 car I'm selling , I call it a no sale because the low end buyers are typically the biggest whiners as they expect a 100% new car for no money.

I'm not in the business, but I've sold my share of used cars. I agree with pretty much everything said, but especially the bolded part.Doubly so for Craigslist buyers.

Granted, I'm guilty of this to a degree. I try to buy more than I'm paying for, but the difference is that I leave the questions to a minimum and leave my expectations low. I'll take a chance when I feel good about a car, but I always inspect it myself, and have only once done a pre-purchase compression test.

Actually, that's worth mentioning. That was on my current winter car that I bought in 2013, a 1995 Subaru. My previous winter car, also a 1995 Subaru, had recurring compression issues which I believe were not uncommon. The car seemed like a decent buy from the ad, and I asked the seller if he'd mind it if I brought along my compression tester and did a compression test in his driveway as a condition of purchase.

He agreed, I ran the test myself, and I bought the car. I may or may not get another winter out of it, but for me, that test (once again, performed by me on the spot) sealed the deal for me.

Bottom line, I like bargains and make informed purchases when I can, but I'm fully cognizant of the fact that I'm still rolling the dice to a degree. It's a matter of managing risks vs expectations.

barry12345 · #11 07-09-2016, 01:27 PM

You could ask for a compression check done at your cost perhaps. Paid for up front. Hard to say what a sellers response would be though.

I as a seller would go along with this. Although in general I am easy to get along with. Also if it is junk I will not sell it anyways.

At the same time I would resist a complete stranger claiming they are interested in the car coming along and doing it themselves.

Generally though most of us go by the general sound of the engine and a check of the amount of blow by. Making sure it does not overheat and the oil pressure is normal. At least these cars have an oil pressure gauge instead of an idiot light. These cars are almost antique status now if not already. You have to normally expect some issues with them.

funola · #12 07-09-2016, 01:38 PM

Doing a compression test on a 300D turbo requires specific tools and knowledge. Are you planning to do it yourself or hire someone? If the seller agrees, do the test. If not, don't blame him since there is some risk involved, things can go wrong like a glow plug electrode can break.

ah-kay · #13 07-09-2016, 05:20 PM

Quote:

Originally Posted by MagicBus

I'm not in the business, but I've sold my share of used cars. I agree with pretty much everything said, but especially the bolded part.Doubly so for Craigslist buyers.

Granted, I'm guilty of this to a degree. I try to buy more than I'm paying for, but the difference is that I leave the questions to a minimum and leave my expectations low. I'll take a chance when I feel good about a car, but I always inspect it myself, and have only once done a pre-purchase compression test.

Actually, that's worth mentioning. That was on my current winter car that I bought in 2013, a 1995 Subaru. My previous winter car, also a 1995 Subaru, had recurring compression issues which I believe were not uncommon. The car seemed like a decent buy from the ad, and I asked the seller if he'd mind it if I brought along my compression tester and did a compression test in his driveway as a condition of purchase.

He agreed, I ran the test myself, and I bought the car. I may or may not get another winter out of it, but for me, that test (once again, performed by me on the spot) sealed the deal for me.

Bottom line, I like bargains and make informed purchases when I can, but I'm fully cognizant of the fact that I'm still rolling the dice to a degree. It's a matter of managing risks vs expectations.

Doing a compression test on a gas car is more straight forward then a diesel. No heat shield to replace ( unless it is done from the GP hole ), no injector line to remove etc. it is a 15 minutes job compared to a 1+ hr job. Some V6 front wheel drive may take longer. if I were the seller, I would decline. If I were the buyer then I will look for tell tale signs and take my chances.

97 SL320 · #14 07-09-2016, 07:31 PM

To expand on the low end buyer. If someone comes to me says the right things and has the proper attitude, I'll make sure the down on their luck person gets the best value.

It's the "I gots no money because I have poor planning skills" person I avoid. Typical signs are lotto scratchers in the car , smokes / chews , has anything more than a basic cell phone, the "I should not have to work for anything because the world owes me" attitude.

There is a way to do a non invasive compression balance test. Place a clamp on amp meter attached to a recording oscilloscope on the battery cable and crank the engine. You should see a current spike for each compression stroke. On a 5 cyl if you see a low spike every 5th place, there is a cylinder with lower compression than the other ones. This won't tell you how low however.

leathermang · #15 07-09-2016, 10:04 PM

Quote:

Originally Posted by 97 SL320

..... You should see a current spike for each compression stroke. On a 5 cyl if you see a low spike every 5th place, there is a cylinder with lower compression than the other ones. This won't tell you how low however.

So you are measuring the load on the starter ?