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Miata rough running

1997 red 5 spd Miata running fine with plenty of power before I changed the timing belt.

Still running fine, but within 100 miles of timing belt work, Check Engine Light came on. Scanner at first place I took it to showed idle circuit problem. Tech spotted loose clamp on intake end of snorkel. He reset the computer and tightened clamp.

Car running fine when red light came on again about 50 miles later, but not running well at all after intake manifold cleaned out. Now it runs terrible but only at certain speeds and loads as described below, but CEL does not come on. We drive it when we think we can stand it.

Had it to the Mazda dealer in Brandon last month. Tech drove it and found that, yes, it did feel as though someone were turning the key off and on when driving 30 mph in 3rd gear. Also problems in 4th at around 42 mph, 2nd at around 14 mph, but 30 in 3rd is the worst. Seems to be related to grade of road too. If it is pulling up a central Florida “hill,” it is more likely to run steadily.

Like the folks at Dossey, tech at Brandon Mazda dealership also suggested just driving it, hoping that the red light will come on so that computer can provide a clue as to what is wrong.

I have to say that I was surprised. I expected the dealer to be able to hook it up to some sort of scanning equipment, see what was going at various speeds and various load conditions (30 mph in 3rd in particular), then diagnose and fix the problem. Nope, not able to do that. Just drive it until the check engine light comes on.

Folks at Firestone, where I buy most of my tires, suggested that cleaning solution used to clean intake manifold might have damaged a sensor in the intake.

Is there a sensor in the intake that could be causing all my problems?

Who cleaned the intake manifold? How did they clean it? I think if the problem was related to the timing belt job the car wouldn’t run properly at all and would idle poorly.

Whey you had several things done to the car in a short time frame it becomes difficult to pinpoint the problem, but I’m feeling the intake manifold cleaning did more harm than good. There are sensors all over the car, especially involving the fuel monitoring systems and pollution controls. My guess is your fuel mixtures are off because the manifold was damaged or is now partially blocked in some way interferring with the airflow. Or, a sensor measuring the airflow (MAP sensor) or fuel delivery was damaged. The cleaning process could have messed up the fuel injectors.

Whoever did the manifold cleaning screwed it up. The question is; is the shop or person expert enough to make it right? I suspect not, if it was a chain quickie lube joint, or a national tire chain store, god forbid a SEARS!!!

If you have a good mechanic you trust go to him and get an opinion. A Mazda dealer is likely to want you to open your wallet and dump all your money on the table and they’ll figure out how much to take you for to get your car running again.

You did not say that you checked the work on the timing belt. You might be surprised at what you see.

To check if engine timing is off, have your mechanic check the timing between the crankshaft and camshaft by pulling the cam sprocket plastic cover off, and checking camshaft index mark against the top dead center mark on the crankshaft. This check is NOT an ignition timing check.
Check the fuel pressure with the engine under load. The spark plug wires may not be on correctly. (They may LOOK ok).
Change the fuel filter.
If the spark plugs and wires are antiques, donate them to a museum, and put new ones on.
Check the MAF (or, MAP), idle air control valve, and the throttle position sensor. And, check the wiring, and connectors, to all of them.

"If the spark plugs and wires are antiques, donate them to a museum, and put new ones on. "

And on this car plug wires and plugs wear twice as fast as most cars. They are a common cause of these kind of problems.

Thanks for all of your suggestions.

I should have mentioned that when I did the timing belt, I replaced the plugs, the water pump, the thermostat, the radiator hoses, the PCV valve, the oil, the oil filter and the air filter. I also flushed and replaced the coolant.

I guess I could have replaced the spark plug wires too, but I replaced them about 30,000 miles ago and the problem doesn’t seem like a miss to me. Again, Miata was running fine before and after timing belt replacement. While the CEL came on, it really didn’t start running crappy until the cleaning of the intake manifold, and the CEL has not come on since that manifold cleaning service. It runs badly only under certain load conditions, especially 30 mph in third gear, on a level road or a slight grade downhill. Under these load conditions, it feels as though someone is turning the key off and on. Depressing the gas pedal slightly seems to make it come back on, but don’t be taking taking a sip from your coffee cup at that moment.

I bought fuel filters for both my 2002 Miata and this troublesome 97 but have not installed either yet. I must say that I’d be thinking the new fuel filter would solve the problem if it were bogging down under heave acceleration, but it seems to be having this cutting out or fuel starvation problem at relatively light engine loads (especially bad a 30 in 3rd).

I’m confident the spark plug wires are going to the right plugs.

I’m confident that I got the timing marks right when I installed the timing belt. I’m thinking it wouldn’t be running well at any load if I had mucked that up.

I guess I’m going to bite the bullet and remove and clean out the intake manifold myself. But I might try replacing the spark plug wires and fuel filter first.

Thanks again,


" I guess I could have replaced the spark plug wires too, but I replaced them about 30,000 miles ago and … "

For a Miata 30,000 miles is like 60 - 80,000 in most other cars.  The Miata fires twice as often as most cars.  It is an odd thing and it "waste" half of the sparks, but that is the design and that engine really runs nice with everything is as it should be.