Misfires and faltering at high RPM's.

Can a bad battery cause a misfire?

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We messed up slightly with our kit car. We Assumed that the base MX5 ran well when we got it, and we also assumed that having rebuilt the whole thing into an Exocet that it would still run. When we started off for the IVA, we found out this wasn’t the case. The car faltered at higher RPM’s, dropped back off of the power and refused to go over 3500. It was as if we had hit the limiter.

As long as you held the car at 3000 rpm, she seemed fine so we drove her back home gingerly, through the Blackwall tunnel keeping to about 50. This meant being overtaken by everything, even the trucks. Which in a topless little car with no wind protection is scary as hell.

Why such a slow trundle back? Our problem was a misfire.

Can a bad battery cause a Misfire?

First things first then, clearing up a few articles I found online. No. The running power for your engine is provided by the alternator. If your alternator is healthy (Connect a Multi-meter to the battery when the car is running, you should get an output of about 14v to show the battery is recharging) The battery may be used to smooth out high draw but signs of a poor battery are most likely to be seen when you struggle to start the car.

What is a misfire A Misfire is caused by the car struggling to properly ignite the fuel in the cylinders properly. A cylinder has very much the same requirements as a regular fire, you need Fuel, Air and Heat (Spark) in order to get ignition. If one of these is failing, then the engine will struggle, particularly when it gets to higher revs and the load on these parts is unable to keep up.


Ignition Ignition is provided for the engine by the spark plugs, which are fed via HT leads either from the ignition coils or a distributor, depending on the age of the car. All of these items can fail over time, with some cars having known weak points depending on their layouts and service schedules. Our RX8 has notoriously weak coils which are located too close to the engine and fail early due to the heat they are exposed to. They are also often replaced by low powered aftermarket parts during services as the originals are so expensive.

HT Leads may also fail over time as a result of heat from being on top of the engine and the high voltage loads they are exposed to.

But the more common point of failure is the spark plugs, which can require replacement every 10,000 miles for cheap ones with the more expensive “Iridium” plugs claiming to last up to 60,000 miles. Mazda’s own maintenance schedule states these need to be inspected every 27,000 miles, an inspection would involve checking that the plugs are not overly corroded or covered in deposits and that they are still able to deliver a solid spark. These plugs are clearly older, are heavily stained with oil and have seen some mileage.


Spark Plugs can be tested by removing the plug, plugging it in to the HT lead back into the plug, earthing the body of the plug against the engine and turning the car over, which should generate a solid spark across the points. This will also test that the HT leads are providing a solid supply of current to the plugs.

More extensive testing can take place of the HT leads using a multi-meter to test resistance, which should be between 10-25,000 Ohms, although the exact amount will depend on the type of leads you have, with Carbon leads reading a little higher and copper leads being lower.


Finally, Coils or distributors. Ignition coils work by having two interwoven coils of wire, one wrap of coils having more windings than the other. These take the input current and increase it substantially so that it can deliver the required voltage to the spark plugs. The firing order is handled by the ECU to ensure the right cylinder fires at the right time. Testing these requires a multimeter and testing the resistance between the primary and secondary coils (Around 0.4 to 2 ohms) and then the resistance between the secondary coil and the “Tower” (Bit where the HT lead connects) (6 to 15 Ohms) Obviously these will vary depending on the spec of your car.

On the MX5, there are multiple coil packs which are “3 pin” coils as the specification changed part way through the Mk2 models, so make sure to order the right sets.

Distributors simply have a rotating arm which makes contact with a metal element, with the arm contacting the right point to fire the spark plugs in the right order. These can be tested visually by ensuring the rotating arm is contacting all points cleanly, though its worth noting there was never a distributor on an MX5.


The MX5 fuel rail has been covered elsewherein our posts as we replaced all of the fuel lines. For the purposes of a misfire the issue could be a lack of fuel at high revs or poor fuel delivery. Lack of fuel could suggest possibly a poor fuel pump, clogged fuel filter or if you have replaced all of your fuel lines with flexible ones, that you have a kink in the hose.

You can test this by inserting a pressure gauge into the fuel line and checking that you are getting somewhere around 45-50 psi. Make sure to check the pressure after the filter and before, if pre-filter is fine, you may need a new filter.

Fuel delivery is also important, if fuel is getting to the injectors correctly, but the injectors are clogged or not generating a clean spray pattern, then the fuel will not vaporise properly and will not ignite.

Testing an injector involves a fair bit of setup, and a special tool, which you can see below with the lit up 4 on.


This tool replicates the engine firing the injector, sending very short bursts (300ms) of current to the injector to make it fire. For testing, fuel has to be pressurised so that the injector can fire out, replicating the setup in the car.


Ignore that I am spraying fuel over the bench. You probably want to put a cup under them when testing….

You are looking for a good spray pattern, but depending on your injectors the expected pattern will vary. Some injectors have a single hole, others have multi Hole and another set has an “Annular Orifice”. Ours are apparently “Multi hole with two spray” injector, are a google of the part number (195500-3310) finds you some suppliers making after market parts who call these “Large quantity” jets and suggest that the split spray is as intended. If you are testing these yourself, make sure you know what your spray is meant to look like.

Another option is to test them all and look for one which is inconsistent. If one is just dribbling fuel out, or only has a single jet when the others have two well, I’d say thats bad.

Injectors can be repaired and cleaned, small filter baskets at the intake can be replaced as can seals internally, including removing any debris that got through the filter if needed. As a note, whenever I have removed an injector, I seem to break at least one of the top seals which splits when the injectors go back in. These are a standard sized metric seal, available in most kits, and given how much hassle it is to take everything apart again to refit them, I just replace these every time.


Even if you get the right fuel, air and heat into the engine it needs to stay in the cylinder. Poor compression usually implies that a valve or piston ring is loose and letting something out. By the time the spark fires, there may no longer be the optimum mix left.


A Compression test uses a pressure gauge in place of a spark plug to test how much pressure builds-up in the cylinder. If the cylinder has a leak, it wont compress as high. To test this, remove the crank position sensor (this prevents the engine from firing) remove all spark plugs and HT leads and then insert the tester into the cylinder top. Turn the engine over 7 or 8 times, and the gauge should stop at its highest reading.

As with the injectors, you want a consistent reading across all cylinders. If you dont get this, one cylinder may have a failed piston ring or loose valve. Two cylinders in a row being down-on compression might also suggest a head gasket issue with the seal in between those cylinders being at fault.

Ours ran at about 175 across each cylinder with-other users online reporting ranges of between 160-190 psi on perfectly running engines.


So having tested all of these things and found all of them to be either basically fine, or we ended up replacing them just because they looked like they might have been overdue. We assembled the engine again, fired her up and… nope. Still getting a misfire.

Cue some swearing.


We did discover what the issue was, and it was so simple as to be infuriating. The PCV Valve.


A valve which sits on the top of the engine to release pressure which builds up in the rocker cover. When the pressure gets high enough, the valve opens and dumps the pressure into the inlet manifold, back into the engine itself. Except ours wasn’t sealing. At all. It was just pissing air (and oil) into the inlet manifold all the time The inlet was caked in a solid layer of oily sludge. When the engine pressure increased at higher RPM, so did the oil flow rate, pushing the now leaking oil directly back into the engine which was firing away. A bit of oil into that fuel/air mix and she would misfire. Its hard to tell if this was the cause of the issue, or if everything together running at just a little under par was causing the problem, but we are now there. We think.

Again though, because we cannot drive the car until the IVA is booked again, we dont know how she will fare on the road. Suffice to say that running her up in the garage and sitting at 6000rpm for 10 minutes has pissed the neighbours off a bit, bit we still dont know for sure if its fixed. Tyres crossed, thats the end of the problem.


Welcome to Twits with Spanner. A blog following two chaps as they attempt to build an Exocet kit car with bits from a donor MX5, spanners and lots of hope. Read more about us