how to diagnose intermittent engine problem


Asked by Mar 15, 2010 at 04:31 PM about the 1992 Mazda MX-5 Miata Base

Question type: Maintenance & Repair

engine stumbles under medium or hard accelleration in all gears, but not always, sometimes it runs fine, it always runs fine when cold. changed plugs,wires,air filter and pcv valve,but problem persists

7 Answers


Vacuum leak? Check fuel pressure and filters (line and tank)?

3 of 3 people found this helpful.

Thanks, I just changed fuel filter (line). Is there a second filter in tank? Do I need a special tool to check fuel pressure? I saw something where you put a t in the line up by the engine and a pressure guage (which I don't have) and then re-hook up the line. I have just cleaned almost every electrical connection that I can find that is related to sensors under the hood. Problem persists, but only about half of the time now. First after engine warms up and has been running fine, the check engine light comes on, then idle goes up from 800 to 1800 then upon engaging any gear and trying to accellerate, car stumbles and breaks up until up to speed, then problem goes away for a while. Intermittent problems are the hardest to fix. I get a code 9 (thermistor) on the diagnostic check. I have had the car a couple of months and the last owner replaced the oxygen sensor with an aftermarket one and the check engine light would always flash off and on, but the car always ran fine until a couple of weeks ago when the racing idle thing started accompained by the stumbling. I am sure others have had this same problem. how have others fixed it?

1 of 1 people found this helpful.

Yes there is a second filter around the fuel pump, you may be able to remove and clean it if needed. It's number 9 in the drawing.

4 of 4 people found this helpful.

lookds like a useful site. its hard to get good diagrams of engine parts. I have an aftermarket o2 sensor and it looks like a mazda part is only $35. Now to find a thermistor and figure out how to get that in. I am at a disadvantage having pretty big hands. I can barely see behind the engine, much less reach it. I wonder where is the best place to get a temperature sensor. I will try to clean my fuel pump filter and then I guess I will go back to replacing parts until something fixes this.

3 of 3 people found this helpful.

Well don't you hate it when you have a car problem and you search the internet and find someone who had the same problem as you and you see all of the things they tried, but when they finally get it fixed, they don't tell you what it was? Well I am happy to report that my car is now fixed. It runs perfectly. The problem was...... faulty ecu sensor for coolant temperature. It had an intermittent fault, that was hard to diagnose. $20 Auto Zone. I had to take out the coil pack to find it. I knew it was there on the back of the engine, but I couldn't see it, and my giant hands couldn't get back there until I pulled some stuff out of the way. The green wire clip that holds the electrical connection together is secured by a small metal (square) ring that had to be pried off to get it apart. Then the sensor removed like a spark plug with a socket wrench. That's it. Happy motoring!!!

11 of 11 people found this helpful.

Thanks for posting the solution. My '92 had exactly the same symptom so I replaced the coolant temp sensor on your suggestion and it runs great so far.

1 of 1 people found this helpful.

Wondering if this is my issue...been to three mechanics and they all say its not the transmision its a electrical issue with the computer not telling engine when to downshift.1997 with 65000 miles on it,automatic and it itermittently will just start revving and won"t downshift.Is that what yours was doing?

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