neosunrise

Rough Idle After Replacing The Timing Belt

8 posts in this topic

My 2000 Mazda 626 has about 68.5k on it. It ran great and smooth. With that said, I thoutht I shoud get the timing belt replaced. The other day, I asked a car care shop to replace the timing belt, water pump, and other two belts. They promised to get it done in two days.

However, I hadn't got their call on the 3rd day so I just called them. I was told the car was ready for pick up. I arrived there and tried to start the car but I didn't make it - when I cranked the car, I can hear the starter sound but the engine didn't start. It started after my second try with giving a bit gas.

The new problem was, the car shaked intermittently and the idle was rough - the RPM varied from 500-750 and never got steady. I decided to leave the car there and let them inspect it (They admitted that there was something different between then and the time when I left the car).

Today, I got their call and was told that the timing belt was installed correctly - they suspected that my old timing belt was loose (actually it looks good and the water pump also looks new) so the timing was a bit off. But since ECU was used to that "off-timing", I didn't notice any problem.

I am just wondering, is that true? I never heard anyone claiming that replacing time belt has to do with ECU. I even doubt if they broke something else on my car.

Share this post


Link to post
Share on other sites

Yes it is possible that the ECU didn't correctly adjust for the new timing. It's not possible that the belt was loose if your car ran fine. A loose belt even if slightly loose has absolutely no bearing on the matter as long as it stays in time. That statement is not accurate. Either it's in time or it's not. This suggests that they didn't reset the ECU, missing some kind of timing adjustment like the TPS, Air Adjust Screw, Throttle Stop, etc... Setting the idle on the 5th gen is very tricky to the point that you only want someone with professional equipment to do it (my opinion). Also there is spring tensioner which needs to be properly set and if it's not then the belt can jump time causing more issues than your original service was set to solve. If it was my car I wouldn't blame them too much for not getting it right the first time. However they shouldn't have let you pick up the car without test driving it first... that is unprofessional.

Share this post


Link to post
Share on other sites

Yes it is possible that the ECU didn't correctly adjust for the new timing. It's not possible that the belt was loose if your car ran fine. A loose belt even if slightly loose has absolutely no bearing on the matter as long as it stays in time. That statement is not accurate. Either it's in time or it's not. This suggests that they didn't reset the ECU, missing some kind of timing adjustment like the TPS, Air Adjust Screw, Throttle Stop, etc... Setting the idle on the 5th gen is very tricky to the point that you only want someone with professional equipment to do it (my opinion). Also there is spring tensioner which needs to be properly set and if it's not then the belt can jump time causing more issues than your original service was set to solve. If it was my car I wouldn't blame them too much for not getting it right the first time. However they shouldn't have let you pick up the car without test driving it first... that is unprofessional.

Thank you so much DJ. I will bring your comments to them and we will see if they can get it fixed! Will keep you posted.

Share this post


Link to post
Share on other sites

Timing is most likely off. DOHC engines are notoriously difficult to time correctly, because the process of tensioning the belt itself can pull any one of the points out of alignment. So you have to set things slightly "out" and then allow the tensioner to pull it "in"... Once I figured that out, my 626 actually had _more_ power after the new timing belt than it had ever had before, even before the old tensioner failed. I lucked out!

One BIG warning: If they have to go back in, make sure they replace the tensioner spring. The tensioner WILL fail eventually, and if they re-used the old one you're just going to have to spend more money down the road.

Share this post


Link to post
Share on other sites

snailman153624 and lotherius, Thank you so much for your messages. I will let them check the timing again tomorrow.

Share this post


Link to post
Share on other sites

OK guys, I just picked up the car and did several test drives. Everything looks good but I noticed something weird (I am not sure if it was the case before the timing belt replacement. I can't remember.).

1. After I start the car, if I press the brake pedal, the RPM drops to about 500 and then goes back to 700 in less than a second. This only happens to the first press after the car is started.

2. When idling, if I turn the headlight (radio, fan, window seem fine. They don't affect the RPM) on, the RPM also drops a little bit (not as much as case 1, probably just 50 difference) and goes back to 700 in a second.

I am just wondering if this is normal? The car drives smooth and has no peoblem on the road. It also has a stable idle if I don't turn on the headlight.

The work they did to my car:

1. replaced timing belt and water pump, as well as the tensioner spring.

2. replaced two accessory belts.

3. refilled 1 gallon coolant.

4. replaced four spark plugs and did a computer tune-up.

The total charge was $420, which I think is reasonable for all the work.

Your comments are appreciated!

Share this post


Link to post
Share on other sites

Ground wire on the passenger side motor mount not getting a good connection?

They did have to remove the ground to get the motor mount off, right?

I would check that ground out first, then move to the battery terminals.

Just my 1 1/2 cents.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now