Car runs well, idles smoothly and pulls cleanly. Has fresh oils and filters, including air filter.
At around during 2000 rpm when coasting there is a gentle hesitation. Increase the throttle and it pulls away cleanly, but running at a constant revs I think it is dropping a cylinder intermittently.
I presume this is electrical, and haven’t dived in yet to change plugs - thought I would ask first.
Are there any gotchas? Is it simply a case of pull off the engine cover off and you get access? Are the coils on the plug caps, or are there separate HT leads? Any usual suspects to look for first?
Car is a 2008 2.0 sport, 72000 miles and I don’t know its service history, so possibly on original plugs, or possibly replaced relatively recently.
You say “when coasting”, to me coasting is with your foot off the throttle completely, overrun. Or even with the transmission in neutral. Is that what you mean or do you mean “cruising”, light constant throttle?
I’ve known a faulty ABS sensor to cause a hesitation while cruising before. The DSC module detects wheel slip when it isn’t actually occurring but the PCM backs of the throttle slightly to reduce engine torque.
It was a pig to diagnose, it wasn’t until it got bad enough for the ABS light to come on a couple of times that I realised the relationship and cause.
Thanks Robbie, yes you are absolutely correct, it is with part throttle, running at constant or gently decelerating speed. Not overrun coasting.
The ABS suggestion makes sense, but suspect as I only seem to notice this at c.2k revs that it is electrical. If the ABS was doing this, I presume it would happen at all revs (but maybe at 4k it is less noticeable).
I’d be interested to read the live data of each wheel speed when it’s happening to see if one of them is erratic. Not sure if you can get an app and a EOBD dongle to do that or not.
I get something similar to this when going down hill on the overrun and just barely touching the accelerator to moderate speed. It feels as if the transition from fuel cut to fuel on is not being handled very well and the car stumbles slightly. I notice it more with cheap petrol. Several stateside posts on this condition suggesting throttle body cleaning and / or spark plug change may help
Cleaned the throttle body, gingerly! Read a few posts about easy to damage body coating, so didn’t wipe or scrub, just a few good sprays with cleaner and soaked up the residue with a rag. Opened the butterfly by holding the throttle pedal open with a plank and sprayed again.
It is not cured, but I think there is an improvement. Might be wishful thinking.
I will change the plugs in a few days and hope that cures it, but it is so slight I could easily live with it. More worried that it is a issue that will get worse due to a failing component.
Did a long drive at the weekend and could feel the issue whether the DSC was switched on or off, so I think that rules out a wheel speed sensor issue?
Read the posts, I’m no expert re engines but could it be old fuel? Assuming you’ve just got the car re not knowing service history.
I’m about to put mine back on the road from next month, don’t obviously know yet if it’s going to run a little rough, I have around 3/4 tank of two month old fuel to use up.
Good call to go to the plugs next, if they haven’t been changed at around 64k miles they are very much due.
Before you go too deep and start pulling things apart and spending your hard earned, pull the top engine cover off and let the engine tick over either in a dark garage or at night time, keep your eyes open for tracking (sparking) around the plugs and coil packs.
Start at the simple/cheapest things first.
I don’t think 2 months will have much impact on fuel, since the lockdown most of us have probably got fuel that old in our tanks. Mine was last filled in March and I have half a tank left. Still goes great on it.
Thanks Reggie. It had been fitted with shorter reach plugs by the previous owner. Looked fine when you took them out, but when I got new plugs there was an obvious difference. I put a post on here seeking help - it is a bit scary putting longer reach plugs in as the damage could be catastrophic if they were wrong.
But when I was reassured that the new ones were correct, they were fitted and instantly cured the issue.
Must admit that when I saw mileage and car just sold I did wonder if someone had changed the plugs at the recommended mileage and put a cheap set in to sell the car.
Simple things first…
I think the wrong plugs was just a mistake. Weren’t cheap plugs, nice NGK’s , just wrong length. In a lot of ways it was re-assuring that the previous service had been done as I had no history at time of purchase.
OK, not good that a mechanic had fitted the wrong plugs, but I would assume oil and filter at the same time, and even if the wrong oil was used, that would be a lot better than 20odd thousand on old oil.
I got the Mazda service history printed out, and it stopped (from memory) at around 50k, which meant worse case for me was car had not been serviced since then (I. E. For 22k miles), but the wrong plugs in there suggests a muppet did a 60k service of some sort. So when I gave it a birthday at 72000 I am hopeful that it has effectively had a full service history, just lacking the documentation.
Anyhow, that was over 4000 miles ago and the engine has been great since.
I maybe a cynic but there are some on line sites which list the “wrong” non iridium plugs for the 2.0 NC at approx 1/3 the price of the correct type. Maybe whoever changed your bought those by mistake…At least you have solved the problem and know what is fitted is correct.