The "Ask a Mechanic" Thread

[RedlightG20]

Alright I have two issues.

First is my mom's 2004 BMW 525. When accelerating, the RPMs move up and down very inconsistently. Overall, the car seems to accelerate fine, but the car is slightly jerky due to the quick and small changes in RPMs. It's acting like you're quickly pushing and letting off of the gas pedal, but in fact you're holding it steady. I've only driven it around town on very short trips so I'm not sure if it gets better as the car warms up. Perhaps a faulty or dirty mass air flow sensor?

The next problem I'm having is with my 2003 Nissan Pathfinder. The brakes make a high pitched noise when slowing down at times, but I had it looked at shortly after I purchased the vehicle (I bought it this past November) and they said that it was temporary and would subside after driving it around a bit. Perhaps I need to get them cleaned? Sort of the same issue with the serpentine belts--they make a squeaking noise, especially on cold and/or rainy days. Is there some kind of spray I can use to fix this temporarily?

[Veretax]

Alright I have two issues.

First is my mom's 2004 BMW 525. When accelerating, the RPMs move up and down very inconsistently. Overall, the car seems to accelerate fine, but the car is slightly jerky due to the quick and small changes in RPMs. It's acting like you're quickly pushing and letting off of the gas pedal, but in fact you're holding it steady. I've only driven it around town on very short trips so I'm not sure if it gets better as the car warms up. Perhaps a faulty or dirty mass air flow sensor?

The next problem I'm having is with my 2003 Nissan Pathfinder. The brakes make a high pitched noise when slowing down at times, but I had it looked at shortly after I purchased the vehicle (I bought it this past November) and they said that it was temporary and would subside after driving it around a bit. Perhaps I need to get them cleaned? Sort of the same issue with the serpentine belts--they make a squeaking noise, especially on cold and/or rainy days. Is there some kind of spray I can use to fix this temporarily?

I don't know about the First issue, but with your breaks issue, I would be curious how long the existing breaks have been on there. I know on my Impala for example it does squeal some times as dust will get on the rotors sometimes, but it also will squeal a bit after the pad has worn down. If you are not sure which is the case it can't hurt to have the tire taken off too look to see how it wears. I'm about ready to put a new front set on my car, and the mechanic there said you figure on a front wheel drive car with the weight being how it is that you'll go through two sets of pads normally over the rear, of course thats given the particular quality pads I'm running. If you are running a low lifespan version they could wear faster I'm sure. (granted I'm not a mechanic, but this is what I've been told)

[twa]

Could spark plugs nearing their end of life expetancy cause the car to shimmy some at low speeds when the car hasn't been running for too long?

Yes,and a cheap fix if that is it.

I'm assuming shimmy means rough idle and hesitation.

[twa]

Alright I have two issues.

First is my mom's 2004 BMW 525. When accelerating, the RPMs move up and down very inconsistently. Overall, the car seems to accelerate fine, but the car is slightly jerky due to the quick and small changes in RPMs. It's acting like you're quickly pushing and letting off of the gas pedal, but in fact you're holding it steady. I've only driven it around town on very short trips so I'm not sure if it gets better as the car warms up. Perhaps a faulty or dirty mass air flow sensor?

The next problem I'm having is with my 2003 Nissan Pathfinder. The brakes make a high pitched noise when slowing down at times, but I had it looked at shortly after I purchased the vehicle (I bought it this past November) and they said that it was temporary and would subside after driving it around a bit. Perhaps I need to get them cleaned? Sort of the same issue with the serpentine belts--they make a squeaking noise, especially on cold and/or rainy days. Is there some kind of spray I can use to fix this temporarily?

The BMW I'll pass on

The brakes might need burning in,run it up to 60-70 and do a couple hard braking stops...depends on the pads

The belt you can buy belt dressing to spray on for a temporary fix ,but it won't last...probably a loose belt or worn tensioner.

[Veretax]

Yes,and a cheap fix if that is it.

I'm assuming shimmy means rough idle and hesitation.

I don't think it feels like a rough idle or hesitation, in fact when sitting at a complete stop or in park I don't feel the kind of vibration. I didnt even notice this until I took a friend home a week ago and she said why is your car shimmying. I thought perhaps it was because of the cold weather and under inflated/under warmmed tires, but I've since had the tires all pressure checked and it still does it after that fact.

[twa]

Could be a bad steel belt on a tire.

Try driving 5-10 mph on a asphalt road and see if it feels worse.

You feel it thru the steering wheel or the seat?..if the seat it is a rear tire

[polywog999]

I don't think it feels like a rough idle or hesitation, in fact when sitting at a complete stop or in park I don't feel the kind of vibration. I didnt even notice this until I took a friend home a week ago and she said why is your car shimmying. I thought perhaps it was because of the cold weather and under inflated/under warmmed tires, but I've since had the tires all pressure checked and it still does it after that fact.

This can be very serious. Have your tires and wheel barrings checked. It's usually free or small charge. One of your tires could blow!

[Veretax]

Could be a bad steel belt on a tire.

Try driving 5-10 mph on a asphalt road and see if it feels worse.

You feel it thru the steering wheel or the seat?..if the seat it is a rear tire

I can feel it in the wheel when driving, and when sitting in the passengers seat the car has a wiggle to it almost.

This can be very serious. Have your tires and wheel barrings checked. It's usually free or small charge. One of your tires could blow!

You know, now that you mention it they did mention I might have a bearing starting to go bad at inspection. I had forgotten about it. However, I also know that my tires are about due for replacement. I was hoping to get past the winter months before I changed them as I also want to get the brake pads changed here in February. hrms... thanks for the advice. Fortunately I don't drive very far or much so the car sits most of the time, but I will see about getting it checked.

[Springfield]

Better brand, Chevy or Jeep? I'm FINALLY looking to get a newer model Jeep or Chevy. I'm looking at the 05-07 Grand Cherokee's and Trailblazers. I'm wondering which one might be more reliable. Any advice would be much appreciated. I have a 96 Grand Cherokee right now and the thing has been an absolute warrior for me. It's at 165,000 right now and it could probably last for a couple more years.

Chevy imo.

Chevy imo.

Trailblazer>Grand Cherokee.

[Springfield]

Could spark plugs nearing their end of life expetancy cause the car to shimmy some at low speeds when the car hasn't been running for too long?

They could... but there are several other things that come to mind before spark plugs. Ignition wires, ignition coils, idle air control valve, mass air flow sensor just to name a few.

[twa]

belt separating on the tire will definitely feel like that

[Springfield]

Alright I have two issues.

First is my mom's 2004 BMW 525. When accelerating, the RPMs move up and down very inconsistently. Overall, the car seems to accelerate fine, but the car is slightly jerky due to the quick and small changes in RPMs. It's acting like you're quickly pushing and letting off of the gas pedal, but in fact you're holding it steady. I've only driven it around town on very short trips so I'm not sure if it gets better as the car warms up. Perhaps a faulty or dirty mass air flow sensor?

Mass air flow sensor is a good place to start. I'm not too positive about BMW, but you should be able to unplug the MAF sensor connector to verify. It will cause a check engine light and may run rough for a second, but when it realizes that the MAF sensor is not within range, it will go to a base (predetermined) A/F map.

Other fairly common issues that could cause that symptom are ignition coils and cam sensors. There isn't a very easy way to check those without diagnostic equipment.

Good luck. Diagnosing BMW's can be tricky sometimes.

The next problem I'm having is with my 2003 Nissan Pathfinder. The brakes make a high pitched noise when slowing down at times, but I had it looked at shortly after I purchased the vehicle (I bought it this past November) and they said that it was temporary and would subside after driving it around a bit. Perhaps I need to get them cleaned? Sort of the same issue with the serpentine belts--they make a squeaking noise, especially on cold and/or rainy days. Is there some kind of spray I can use to fix this temporarily?

For the brakes, it sounds like the rotors are rusty. Does this happen after it's been sitting for extended periods, especially on rainy or humid days? If so, that's what it is. The noise will go away after the first few braking applications as the pads clear the rust off of the brake rotors.

If it still happens after the first few braking applications (assuming the pads are in good shape), then I'd say a low quality brake pad. No safety issues, just noise.

Either way (assuming the pads are in good shape), it sounds like the last time the brakes were replaced they used lower quality parts. Low quality parts are less expensive but normally result in unwanted brake noise.

As for your belts. They either need to be tightened or replaced. You can bank on that one.

[Springfield]

I don't think it feels like a rough idle or hesitation, in fact when sitting at a complete stop or in park I don't feel the kind of vibration. I didnt even notice this until I took a friend home a week ago and she said why is your car shimmying. I thought perhaps it was because of the cold weather and under inflated/under warmmed tires, but I've since had the tires all pressure checked and it still does it after that fact.

Could be a bad steel belt on a tire.

Try driving 5-10 mph on a asphalt road and see if it feels worse.

You feel it thru the steering wheel or the seat?..if the seat it is a rear tire

This can be very serious. Have your tires and wheel barrings checked. It's usually free or small charge. One of your tires could blow!

I can feel it in the wheel when driving, and when sitting in the passengers seat the car has a wiggle to it almost.

You know, now that you mention it they did mention I might have a bearing starting to go bad at inspection. I had forgotten about it. However, I also know that my tires are about due for replacement. I was hoping to get past the winter months before I changed them as I also want to get the brake pads changed here in February. hrms... thanks for the advice. Fortunately I don't drive very far or much so the car sits most of the time, but I will see about getting it checked.

belt separating on the tire will definitely feel like that

The more I read, the more I agree with twa. It does sound like a broken belt in the tire. Could also be a bad bearing, but I'd look at the tires first. It's the easiest thing to do.

Run your hand across the surface of the tire. If it pokes you with steel, then you need to have it replaced. That would be the steel cords coming through the tread of the tire. That's not the point though.

A broken belt in a tire will feel like a bulge in the normal roundness of the tire. Some are worse than others but if you run your hand across it, all the way around, you can normally feel it... especially if it's causing a vibration at low speeds.

So...

Run your hand across the tread of the tire (the part that makes contact with the ground). Does it feel flat, accounting for the normal curvature? If there are any bulges or bubbles, there is something wrong.

Here's a picture... The tread of the tire should be flat, not bulging as it is on the left side.

[Veretax]

They could... but there are several other things that come to mind before spark plugs. Ignition wires, ignition coils, idle air control valve, mass air flow sensor just to name a few.

They recently recalled and replaced some of the ignition wiring in my Impala, but I'm not sure how much was replaced, but I believe this issue was going on a bit before then. (I did notice immediately after it that the engine seemed to run a bit smoother and quieter though).

[RedlightG20]

Mass air flow sensor is a good place to start. I'm not too positive about BMW, but you should be able to unplug the MAF sensor connector to verify. It will cause a check engine light and may run rough for a second, but when it realizes that the MAF sensor is not within range, it will go to a base (predetermined) A/F map.

Other fairly common issues that could cause that symptom are ignition coils and cam sensors. There isn't a very easy way to check those without diagnostic equipment.

Good luck. Diagnosing BMW's can be tricky sometimes.

For the brakes, it sounds like the rotors are rusty. Does this happen after it's been sitting for extended periods, especially on rainy or humid days? If so, that's what it is. The noise will go away after the first few braking applications as the pads clear the rust off of the brake rotors.

If it still happens after the first few braking applications (assuming the pads are in good shape), then I'd say a low quality brake pad. No safety issues, just noise.

Either way (assuming the pads are in good shape), it sounds like the last time the brakes were replaced they used lower quality parts. Low quality parts are less expensive but normally result in unwanted brake noise.

As for your belts. They either need to be tightened or replaced. You can bank on that one.

Thanks for the advice, I'll check back when I find out more!

[RLeVan]

Chevy imo.

Trailblazer>Grand Cherokee.

Unless you've owned both, it's only opinion.

The comparison is like 50:1. I'm not getting into a this is better than that argument.

If you go for a Jeep, go '06 or later, Quadra Drive 2 (Electronic Limited Slip Diffs/Aussie Lockers, Leather Seats).

Most important thing with anything other than Ford or Chevy is make sure their putting the right fluids in engine/tranny/differential.

Forgot to mention, the comments about standard problems with Chrysler transmissions. My '94 4.0L Aw4 only needed a rear seal replacement ($75). Blanket statements are why people are brand loyal. The Jeep newer trannies are still made by Mercedes.

My '94 4.0L is still running strong and my '06 Overland has no problems as well. Still get the occasional chuckle when I gun it getting on the interstate.

Edited January 21, 2010 by RLeVan

[Springfield]

Unless you've owned both, it's only opinion.

The comparison is like 50:1. I'm not getting into a this is better than that argument.

If you go for a Jeep, go '06 or later, Quadra Drive 2 (Electronic Limited Slip Diffs/Aussie Lockers, Leather Seats).

Most important thing with anything other than Ford or Chevy is make sure their putting the right fluids in engine/tranny/differential.

Forgot to mention, the comments about standard problems with Chrysler transmissions. My '94 4.0L Aw4 only needed a rear seal replacement ($75). Blanket statements are why people are brand loyal. The Jeep newer trannies are still made by Mercedes.

My '94 4.0L is still running strong and my '06 Overland has no problems as well. Still get the occasional chuckle when I gun it getting on the interstate.

Of course my statement is only opinion.

Any car will last a very, very long time as long as it's properly maintained. The main problem is that most (and I'd say at least 75%) of the vehicles on the road today are not properly maintained. It's as much my fault, as the service adviser, as it is the owner of the vehicle.

That's one of the problems with the business that I'm in. Most people will jump to the conclusion that the industry as a whole is ripping them off when we tell them that they need more maintenance. For instance, when we say that the differential fluid in a Jeep needs to be replaced every 15K miles, they assume that is too often and just don't do it.

Also, I blame part of the lack of maintenance on the auto industry (Ford, Chrysler, GM, Honda, Toyota, etc) as well. Many have warranties that are 7 years/75K miles, 10 years/100K miles as example. The auto industry advertises their cars as more reliable than ever. They say they don't need as much maintenance. This couldn't be any further from the truth.

[RememberOsaka]

That's one of the problems with the business that I'm in. Most people will jump to the conclusion that the industry as a whole is ripping them off when we tell them that they need more maintenance. For instance, when we say that the differential fluid in a Jeep needs to be replaced every 15K miles, they assume that is too often and just don't do it.

What IS a ripoff is how much the Jeep dealer charges to change the differential fluid when compared to what my local ASE certified shop charges.

[SkinnedAussie]

Yeah, but a Jeep dealer uses genuine Jeep diff fluid, whereas a regular mechanic uses the cheap and nasty stuff.

[HOF44]

I'm looking for a good set of tires. The sizes are;

225/45R18 (Front)

245/45R18 (Rear)

I'm looking for a decent price, all season sport tire. I'll give up a little grip for better tread wear. So far the front runner for me is the Bridgestone Potenza RE 960AS Pole Position.

Seems to be a well reviewed tire. Just checking in here for an expert opinion.

[Springfield]

Yeah, but a Jeep dealer uses genuine Jeep diff fluid, whereas a regular mechanic uses the cheap and nasty stuff.

I don't necessarily agree here. We use BG products. Those fluids are the far superior to any OEM fluid in almost any case. Nothing cheap nor nasty about BG fluids.

[Springfield]

I'm looking for a good set of tires. The sizes are;

225/45R18 (Front)

245/45R18 (Rear)

I'm looking for a decent price, all season sport tire. I'll give up a little grip for better tread wear. So far the front runner for me is the Bridgestone Potenza RE 960AS Pole Position.

Seems to be a well reviewed tire. Just checking in here for an expert opinion.

I'm not a fan of Bridgestone to be honest. I haven't heard as many bad things about them as I have Goodyear, but I haven't really heard much good either.

There are tires out there that will certainly outperform the RE960 for more money, and also tires that will perform just as well for considerably less.

What you should pay attention to is the UTQGL ratings, not what the manufacturer says as far as their life time. UTQGL (Uniform Tire Quality Guide Line?) gives a much more predictable rating on tires as far as treadwear, traction and temperature. I'm at work and don't have access to anything right now, but I know www.tirerack.com has a very good description of what exactly the UTQGL does on their page.

I'm going to list 4 other tires that I'd install on my car before the Potenzas. I'll also give you their treadwear rating according to the UTQGL.

BF Goodrich G Force KDW2: 300 AA A (treadwear, traction, temperature). This tire will be less than the Potenza by about $30. It's also a summer tire, so you may want to steer clear.

Dunlop Sport Signature: 420 AA A. This tire will be less than the Potenza by about $40.

Kumho Ecsta Sport: 320 AA A. This tire will be less than the Potenza by about $60.

Michelin Pilot Sport A/S Plus: 500 AA A. This tire will be more than the Potenza by about $15.

Bridgestone Potenza RE960: 400 AA A. Listing this one for comparison.

All of those tires are available in both sizes.

[RememberOsaka]

Yeah, but a Jeep dealer uses genuine Jeep diff fluid, whereas a regular mechanic uses the cheap and nasty stuff.

My mechanic uses the best fluids. Any other reasons for the disparity in price?

Edited January 21, 2010 by RememberOsaka

[Springfield]

My mechanic uses the best fluids. Any other reasons for the disparity in price?

This is a rhetorical question, I assume.

In the case it isn't...

Dealers charge more per hour than private shops. The difference is often in excess of $20 per hour in labor. The OEM fluid from the dealer is often less expensive though. It isn't enough to even up the disparity between parts and labor though.

[SkinnedAussie]

Fair go guys, I was taking the piss, you know, being sarcastic.