Car

Ask a Mechanic Online and Get Answers to Your Car Questions

Ask an Expert,
Get an Answer ASAP!

Car
This answer was rated:

My check engine light came on so I took it to a mechanic.

He did a compression test...
My check engine light came on so I took it to a mechanic. He did a compression test and said that piston number 3 was at 125% whereas the other three were fine. He said he needed to replace the rings on that piston. In the next breath he said that whenever you replace a piston it means you need to either get a new engine or have the present one rebuilt. He said he has a friend that sells used engines (which I wouldn't get becaue I don't trust used engine from junkyards) and he could get one for me if I liked. I asked him if everything else was fine, why did I need my entire engine rebuilt? He said its required when you replace one piston to just rebuild the engine. Is this true?
Show More
Show Less
Ask Your Own Car Question
Answered in 5 minutes by:
10/2/2013
AMD
AMD, Master Technician
Category: Car
Satisfied Customers: 2,728
Experience: L1 Advanced Level Diagnostics Specialist
Verified

AMD :

Hello and thank you for trusting us with your Car question, My name is XXXXX XXXXX I will be assisting you today.

AMD :

Are you available to discuss your question at time time?

AMD :

If I may ask, what make,model and year vehicle do you have?

Customer:

It's a Chrysler PT Cruiser 2008

AMD :

OK I See. How many miles does your vehicle ahve?

AMD :

Have*

Customer:

124,000

AMD :

OK I See. Was the vehicle running poorly at the time when the check engine light came on?

Customer:

it doesn't run poorly at all

Customer:

it still runs fine

AMD :

OK Do you have a safety inspection or an emissions test coming up soon that you need the check engine light to be fixed?

Customer:

no

Customer:

The mechanic did a compression test and said it was the third piston that was misfiring

AMD :

OK Let me explain something. An engine with a low compression in one cylinder would run very poorly and you can feel a misfire which is often felt as a vibration when you accelerate and extremely poor gas mileage.

AMD :

So my first concern here is the accuracy of the diagnosis, Do you trust this mechanic?

Customer:

A friend referred me to him.. I only met him today

Customer:

He said when he opened the hood he could see the engine vibrating but I don't feel it vibrating when I accelerate

AMD :

OK I See. Well rebuilding or replacing the engine is a major job and is expensive, What I highly suggest you do is to get a 2nd opinion and it would be great to get that opinion from a dealership to be sure that this mechanic didn't miss anything simple because first there are a lot of things that would cause a misfire and there are a lot of other things other than piston rings that would cause a low compression like a leaking head gasket.

Customer:

The only thing I've ever felt was on a few occasions it idled when you startup the engine

AMD :

OK I see. I suggest you get a 2nd opinion from a dealership. Let us discuss the engine options if the 2nd opinion matched the first one, Do you have any questions before we talk about the engine options?

Customer:

I'd like to talk about the engine options

Customer:

Well I did want to know if its true that replacing a piston means you have to do an engine rebuild

AMD :

OK I agree with you that the used engine option is of course not the ideal option. However let me talk about the used engine advantages and disadvantages.

Customer:

Its not an option at all. I don't trust junkyard engines

AMD :

OK I See. then let me move onto your question.

AMD :

The reason your mechanic is telling you a complete rebuild is needed is the fact that the engine is going to be removed from the vehicle, you are going to replace only one piston and one piston ring then put the engine all back together then install it in the vehicle, there's no guarantee that the other rings will not do the same thing a week later then you will have to repeat the same thing all over and the cost will be extremely high.

AMD :

So since you are going to remove the engine from the car and take it apart, might as well recondition the entire engine so now you know for a fact that you will not have any issues with the engine.

Customer:

well he did say that

AMD :

Replacing the pistons and rings require the engine to be taken apart to the bare bones.

AMD :

I highly suggest you do a complete rebuild because we all have experience and trust me you don't want to take an engine apart to replace only one part.

Customer:

he said it would take week to do this

AMD :

That is correct and here's why

Customer:

so then he is correct that a misfiring piston means you need to have the engine rebuilt

AMD :

First when the engine is out of the car, they will take everything apart and send the engine bare block and head to the machine shop to recondition them basically bringing them to a brand new engine specifications. Also the machine shop will recondition the head which is the top part of the engine. Then the mechanic in this time will be getting your parts. Engine parts are not available over the shelf and most of the time will have to be ordered.

AMD :

Once the engine is back from the machine shop which typically takes 3-5 days the mechanic will reassemble the engine outside of the vehicle then install it back into the car.

AMD :

The mechanic is correct that if you have low compression a complete engine rebuild is the best option, However you'd want to get a second opinion to make sure their diagnosis is correct before you make the investment of rebuilding the engine.

Customer:

What else could it be?

AMD :

Also you do want to shop around for the engine rebuild prices. Get a quote from multiple mechanics and compare prices and warranties.

AMD :

If you have low compression, it could also be a leaking head gasket which is a much less major job and doesn't require a full rebuild it only requires the head gasket to be replaced. still a big job but not even close to what a full engine rebuild would cost and typically a head gasket job would take a day or two.

AMD :

I suggest you stop by your local Chrysler dealership and have them give you a 2nd opinion by looking at the car.

AMD :

It will cost you a diagnosis fee but if the original mechanic made a wrong diagnosis then you have automatically saved a lot of money

AMD :

It's very important that the diagnosis be double checked when it comes to a major job like this.

Customer:

The dealerships always charge such high fees for everything

Customer:

But I think you are right better to go the people that manufacture the car for the diagnosis

AMD :

I know, Typically the dealership will charge around 100-150 dollars for diagnosis. But you just need their diagnosis, also have them give you an estimate. Of course they are the most knowledgeable people in your car and I would trust their diagnosis. Also they would give you the highest quality job, so if their price is a bit more than your mechanic but not a lot more I would go with the dealership because they do the best job because they are the specialists in your car.

AMD :

Did you have any other questions for me today?

Customer:

Does the fact that the car has a rough idle (occasionally) at start-up confirm that the first mechanic's diagnosis may be correct?

AMD :

It may be correct given that the car has a rough idle, but if they did something wrong during the compression test and their reading is wrong then it could be a bad injector or a bad coil that would also cause a misfire which is nothing even close to being major like a piston\ring or head gasket. that's why a second opinion is definitely a good idea

Customer:

Would a bad injector or bad coil cause a rough idle?

AMD :

It would cause a rough idle for sure and would cause the check engine light to indicate a misfiring cylinder.

AMD :

Because every cylinder needs fuel from the injector and fire from the spark plug\ignition coil and compression from the piston\ring to run right

AMD :

if any of the three has an issue it will misfire and cause the rough idle and lack of power and the check engine light

Customer:

Also I notice that when I'm stopped at a stop sign or red light the check engine light starts flashing (when its normally steady). why does stopping for a few minutes cause the check engine light to flash?

AMD :

When the check engine light starts to flash this indicates that the engine is misfiring right now. It's just a way to warn the driver that the engine is not running right. if you continue to drive the vehicle like this for a long time like 6 months to a year the catalytic converter which is a part of the exhaust and is a very expensive part of the exhaust will get ruined and also will cause the check engine light to be on even after you fix the current misfire issue.

Customer:

I just thought of something: wouldn't the compression test the mechanic performed have shown if it was a bad coil or bad injector?

AMD :

Typically a misfiring engine recovers from a misfire at higher engine speeds and is more prone to misfire hard under heavy load or at idle when the speed is the slowest.

AMD :

The compression test is the 3rd thing any mechanic would do in the misfire diagnosis because that is the most involved test out there. Typically we start by a method called swapping coils and swapping injectors. For example if we have a bad coil or injector in cylinder 3, if you change this injector or coil with cylinder no.2 then start the car, cylinder no.2 would start running wrong not 3. so this is a very quick way to know if you have a bad coil or a bad injector. If you swap both and it still has number 3 misfiring then we do a compression test which is step 3 after swapping an injector and a coil. if the compression is low then we perform a test called a cylinder leak down test which tells us where is the leak coming from that is causing a low compression number.

AMD :

With the cylinder leak down test we know if it's a head gasket that is leaking or a piston ring or sometimes even a bad valve.

AMD :

So all these steps has to be done in order to accurately diagnose this issue. If your mechanic came back to you within 1/2 hour telling you the car needs an engine, I can tell you with confidence that they are incompetent and dishonest.

Customer:

He went straight to the compression test

Customer:

And called me over so I can see the temperature gauge on the third piston was at 125

AMD :

Well that's the first mistake. You never skip steps in a diagnosis process, because a wrong diagnosis could cost you a lot of money and that is just not right. before you invest all this money you definitely want to stop by the dealership and have them take a look to make sure that the first diagnosis is right.

AMD :

So to summarize I cannot emphasize how important it is that you get a second opinion on this matter before you do anything

Customer:

I will definitely go to the dealership and get a second opinion. I will not tell them what the first mechanic said... I will just tell them my check engine light is on and see what they say

AMD :

Exactly. I would trust what the dealership says more than any other mechanic. Also compare the prices from both the first mechanic and the dealership. Sometimes the price is only a little bit higher on the dealership and is worth spending a bit more.

AMD :

Did you have any other questions for me today?

Customer:

Does an engine rebuild make the car like new despite having 125,000 miles

AMD :

Of course. Regardless of the mileage, once an engine is rebuilt it is basically as good as new because everything vital inside of it is replaced with a new part and\or machined to like new specification.

Customer:

wow so if you decided to get a new engine its cheaper than buying a new car?

AMD :

Well the car has a lot more components than the engine. You still have the transmission, the brakes the suspension and a lot of various components that are still with 125,000 miles but having a new engine is a very big reliability boost at 125,000 miles and would give you the piece of mind that you won't have any engine issues for another 125,000 or even possibly more.

Customer:

very last question of the night.... do you trust the cars at auctions.. not the auctions that are open to the public but the ones open only to dealers

AMD :

I personally have dealt with cars that are open to the dealerships only and I would tell you that I wouldn't take a car from an auction even if it was given to me for free :-) Typically they only give you a few minutes to check a car and even if you go a day before the auction to check the cars you won't know how the car is. Sure you will buy it cheaper but you won't know how good the car is until it's too late and you've already bought it. These days it's rare to buy a good car from the auction. If you are a mechanic then it's OK because you are buying a car to fix, but being a non mechanic it can get expensive very quick and whatever you save buying a car cheaper from the dealer auction you will end up spending fixing the car. I would just buy either private on craigslist or buy from a reputable dealership.

Customer:

Thanks for your advice.. I much appreciate it.

AMD :

You're welcome! I'm glad I could help.

AMD :

A positive rating for my input would be greatly appreciated.

AMD :

Thank you for choosing justanswer.com

AMD
AMD, Master Technician
Category: Car
Satisfied Customers: 2,728
Experience: L1 Advanced Level Diagnostics Specialist
Verified
AMD and 87 other Car Specialists are ready to help you
Ask your own question now
Ask AMD Your Own Question
AMD
AMD
AMD, Master Technician
Category: Car
Satisfied Customers: 2,728
2,728 Satisfied Customers
Experience: L1 Advanced Level Diagnostics Specialist

AMD is online now

A new question is answered every 9 seconds

How JustAnswer works:

  • Ask an ExpertExperts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional AnswerVia email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction GuaranteeRate the answer you receive.

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like justanswer.com/legal
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

I would (and have) recommend your site to others I was quite satisfied with the quality of the information received, the professional with whom I interacted, and the quick response time. Thanks, and be sure that I'll be back whenever I need a question answered in a hurry.

Stephanie PElm City, NC

used your service this weekend with "Trecers" help. thank you ,thank you, thank you. replaced an A/C fan motor. Local Auto Zone had part. $15.00 "tracer" fee and $40.00 for parts, I saved several hundreds of dollers at a shop. i will recommend you and use you in the future.

David L.Richmond, TX

9 dollars, 2 hours of my time, and I drove away. Your diagnosis was right on the mark. Thank you so much.

PhilMarysville, CA

Lurch. Thank you very much. I had real doubts about this website but your promptness of response, quick followup and to the point answer with picture was incredible.

CharlesWalnut Creek, CA

As a single woman, I really appreciate an excellent and affordable opinion.
Thank you Geordie, I will not hesitate to contact justanswer in the future!

SueCharleston, WV

Another great insight to what may be the problem. I will have my mechanic take a look at it tomorrow. Thanks again, Frank...you do indeed know your stuff.

JimCastleberry, FL

The nearest Volvo dealer is 160 miles away. Best $15 bucks I have spent in a long time!

Paul MCorpus Christi, TX

< Previous | Next >

Meet the Experts:

Chris (aka-Moose)

Chris (aka-Moose)

Technician

1,243 satisfied customers

16 years of experience

Ted G.

Ted G.

ASE Certified Technician

1,724 satisfied customers

20 years auto repair experience, ASE Master Tech, Mechanical Failure consultant, Expert Witness

George H.

George H.

ASE Certified Technician

1,537 satisfied customers

ASE Master Tech 15+ yrs, AAS Automotive Technology, Factory trained Asian specialist

GM Tech (Cam)

GM Tech (Cam)

Auto Service Tech

669 satisfied customers

GM Grand Master Technician 2007. 14 years experience.

Amedee

Amedee

Former ASE Master Tech

2,520 satisfied customers

ASE certified tech advanced level specialist. Wisconsin certified emissions state inspector

Jerry

Jerry

Master Mechanic

2,181 satisfied customers

ASE master, 30+ years. All makes and models. Trouble shooter, shop forman, service manager

Richard

Richard

ASE Certified Technician

1,000 satisfied customers

12 years Ford Lincoln/Mercury Jaguar dealership as a technician and shop foreman reparing all makes

< Previous | Next >

Related Car Questions
We have a 2014 Jeep Grand Cherokee with 66k miles on it. We
We have a 2014 Jeep Grand Cherokee with 66k miles on it. We bought it used from a reputable dealer but are now out of warranty. We suspect that we are hearing a hydrolic lifter tick. Several mechanics… read more
vwtech0405
vwtech0405
VW Technician
Associate Degree
8,868 satisfied customers
I have a 2008 Sonata which has a grinding sound coming from
I have a 2008 Sonata which has a grinding sound coming from the serpentine belt area. I'm guessing it is a pulley bearing. It is not setting off any warning lights. Wondering how much the repair might… read more
Ron Z.
Ron Z.
Vocational, Technical or Trade Scho
2 satisfied customers
My daughter has 2003 Sonata. The car is missing the dust
My daughter has 2003 Sonata. The car is missing the dust caps for the service ports on the air conditioning. I have tried everywhere no luck. Any suggestion? Also the low side does it take a special f… read more
ggrote1
ggrote1
Vocational, Technical or Trade Scho
13 satisfied customers
My 98 145000 town car is to slow What can i do to it to make
My 98 145000 town car is to slow What can i do to it to make it faster And still pass emissions test. Can i put A turbo on it and when its time For testing take it off?… read more
Stephen, MBA
Stephen, MBA
Master's Degree
166 satisfied customers
Thinking about purchasing 2005 Yukon w. 115000 miles.
Hi thinking about purchasing 2005 Yukon w. 115000 miles. Service stability and tire monitor messages on. There are aftermarket tires and rims. Should I buy this thing??… read more
Lou P. master tech
Lou P. master tech
Mercedes Benz master tech
Bachelor's Degree
649 satisfied customers
I need to know if the tint on the rear windows was factory
I need to know if the tint on the rear windows was factory installed. … read more
Gabriel Grey
Gabriel Grey
Master Service Technician
Associates
378 satisfied customers
I got a car with a subwoofer in the trunk, and it looks like
I got a car with a subwoofer in the trunk, and it looks like the power amplifier is burned up. How do i go about getting a new one and what do i do to fix this?… read more
eauto
eauto
self employed
High School or GED
74 satisfied customers
I am interested in buying a used Suburu Outback from sell by
I am interested in buying a used Suburu Outback from sell by owner. It is a 2006 with 148,000 miles. He is asking $6,500 for it. He says it has new tires and new headgaskets, has been taken good care … read more
Chad Farhat
Chad Farhat
Owner
Associate Degree
4,446 satisfied customers
I have noticable fuel leaking from the fuel pressure relief
I have noticable fuel leaking from the fuel pressure relief on a 2006 volvo s60 2.5T. There is no CEL and not much difference in performance but the smell of gas is noticable even just standing near t… read more
David Craig
David Craig
OWNER/MECHANIC
Associates
4,420 satisfied customers
Looking for Value of 1991 Chevrolet Silverado 2WD Suburban,
Looking for Value of 1991 Chevrolet Silverado 2WD Suburban, Excellent condition, flawless interior, 96300 miles (mostly annual camping trip then barn storage), no rust/excellent exterior, corduroy int… read more
Eric
Eric
Retired
N/A
14,436 satisfied customers
Dealer said car had a collapsed transmission mount, but the
dealer said car had a collapsed transmission mount, but the car has only 30K miles and there is no vibration from the motor or transmission, so I think I'll wait until there is. … read more
ggrote1
ggrote1
Vocational, Technical or Trade Scho
13 satisfied customers
My timing chain broke on my 2007 Azeri. 165000 miles. Good
My timing chain broke on my 2007 Azeri. 165000 miles. Good tires. Just spent 3000 fixing air conditioning and front axles. Should I fix it or sell it for whatever? Engine damaged? Thanks. Jon… read more
David Craig
David Craig
OWNER/MECHANIC
Associates
4,420 satisfied customers
I replaced my a/c compressor. walmart sell freon gas for $5
i replaced my a/c compressor. walmart sell freon gas for $5 and the autodiscount sell it for $11, freon from autodiscount states that is "dupont". is better the expensive because it will be more coole… read more
Toyota Service
Toyota Service
Toyota Dealership Service & CR Manager
1,344 satisfied customers
I'm looking for a used Toyota or Honda car but not Prius for
I'm looking for a used Toyota or Honda car but not Prius for under $8,000. The car must have a clean title with mileage under 100,000. Besides Craigslist and Carmax, what are some other places I can l… read more
Eric
Eric
Retired
N/A
14,436 satisfied customers
I'm trying to decide if I should keep my car or sell it. I
Hi there. I'm trying to decide if I should keep my car or sell it. I replaced a number of parts over the last four years, but I'm not sure the average life span of these parts. I work from home and do… read more
sprinkles08
sprinkles08
Technician
High School or GED
11 satisfied customers
Thanks for your time. I am selling my car - this weekend -
Hi. Thanks for your time. I am selling my car - this weekend - and the front (beam) headlight went out TONIGHT!!!Obviously, since I KNOW the buyer, I do not want to sell it to them without a headlight… read more
Eric
Eric
Auto Technician
589 satisfied customers
I was in the process of selling my 2004 Mercury Sable and
Hi, I was in the process of selling my 2004 Mercury Sable and went to get the mileage off the dash , I have the title reflecting 83,169.00 miles, but in order to sell the car she needs the miles now, … read more
Kavey
Kavey
86 satisfied customers
What brand are those A/C hose clamps and who sells them? I
What brand are those A/C hose clamps and who sells them? I looked on Autozone and O-Reilly Auto parts website and I couldn't find them.… read more
Toyota Service
Toyota Service
Toyota Dealership Service & CR Manager
1,344 satisfied customers
Disclaimer: Information in questions, answers, and other posts on this site ("Posts") comes from individual users, not JustAnswer; JustAnswer is not responsible for Posts. Posts are for general information, are not intended to substitute for informed professional advice (medical, legal, veterinary, financial, etc.), or to establish a professional-client relationship. The site and services are provided "as is" with no warranty or representations by JustAnswer regarding the qualifications of Experts. To see what credentials have been verified by a third-party service, please click on the "Verified" symbol in some Experts' profiles. JustAnswer is not intended or designed for EMERGENCY questions which should be directed immediately by telephone or in-person to qualified professionals.

Disclaimer: Information in questions, answers, and other posts on this site ("Posts") comes from individual users, not JustAnswer; JustAnswer is not responsible for Posts. Posts are for general information, are not intended to substitute for informed professional advice (medical, legal, veterinary, financial, etc.), or to establish a professional-client relationship. The site and services are provided "as is" with no warranty or representations by JustAnswer regarding the qualifications of Experts. To see what credentials have been verified by a third-party service, please click on the "Verified" symbol in some Experts' profiles. JustAnswer is not intended or designed for EMERGENCY questions which should be directed immediately by telephone or in-person to qualified professionals.

Show MoreShow Less

Ask Your Question

x