Lack of power on M13A engine

For about 4-5 months, I noticed that the engine doesn't have much power. (M13A)

I am not sure if it came after I switched my street tires (13.5kg each, INSA A/T) with a simex profile (about 18kg/each). But still, it shouldn't be much difference.

The consumption is about 17-19L/100km (HUGE, it's like I'm driving a hummer)
The acceleration is very very slow. Only in the 1st gear I'm feeling some power, all others seem very sluggish.
Beside the ABS sensor, there's no error on the diagnosis.

An important thing: even when going down on the road (steep roads), the car slows down - even if it's on neutral !!! (and this is not an engine issue anymore)

What's been changed/investigated until now:
- changed the spark plugs (from old NGK to brand new Denso)
- the engine compression is ok for a 18y old car
- new camshaft sensor
- new crankshaft sensor (very soon)

What should I investigate next (no particular order):
- catalytic converter
- ignition coil
- ignition wires
- lambda sensor

Thank you!

Thank you Lambert for the warm welcome.

I was expecting a lower mpg, but a friend has 12-13L/100km (22mpg), but I'm at 15mpg (19/100).

Initially the tyres were 205/70/R15 insa turbo ranger A/T (13.5kg) and now I'm at 195/80/R15 insa turbo special track (17kg/each).

Both of them are remoulded tyres.
So, the new tyres have a lower footprint compared to the old ones.

I'll take a day to make a test with my friend's tyres, just swap them, but I really think it's not the right direction to go.

Is there a way to check the catalytic converter without going to a mechanic?

An important thing: even when going down on the road (steep roads), the car slows down - even if it's on neutral !!! (and this is not an engine issue anymore)

This sounds as though you have a seized brake caliper, check for heat but DO NOT TOUCH the brake disc after a run.

Hello Ipop and a BigJimny welcome from the Cotswold Hills!

Sounds like your brakes are binding if you are coasting downhill in neutral and finding you are slowing down,
Also is there oil in the diff? Does the diff' or any of the wheels feel excessively hot? Compare the temperatures of each wheel but don't burn your hand in the process

Good morning Lambert

My Jimny passed its MOT yesterday but struggled getting through the emissions test - I tmay need a new cat.

My question is - are there any recommended suppliers/brands of aftermarket cats that wont break the bank?

I assume the el-cheapo aftermarket one lasted ok did it?

Binding brakes will feel more loose when hot. Almost not binding as things are real hot. Also they should be pulling to one side when cold, and the other side when braking when hot as the hot brakes will work badly.
If really hot you will also damage the bearings and gaskets and seal rings.
And brake fluid.

I found out my cat was broken from a small rattling noise coming from the exhaust manifold. Listen to it through a stick and found the problem area. just press a wooden stick to different areas of metal on the engine, and listen to the end of the stick. You will be able to isolate sounds that way. If there is a rattle inside the manifold the cat might have gone to pieces and those pieces will travel down and damage the next cat as well.

When i changed my cat I went from driving up from our local sub sea tunnel (-287m / 9,6% incline) in third gear barely 80kmh to driving uphill in fifth and had to stop accelerating at 120kmh. Wow the difference, think i have a race car now

I'll see again with the brakes. One thing I know for sure, is that the break doesn't work on the rear-left wheel (for some reason).
Another thing: the mechanic told me that he's gonna fix it, but first I need to change rear axle with a better one (it's cracked a bit).

Now after reading so many replies, it may be possible to the oil inside the axle/groups to be almost gone (no noise comming from it) or the oil is more dense than before. And this would cause a break, I think.

A quick oil replacement for the rear axle group would do the trick?

And btw, the cat isn't glowing red (hot) after driving a lot (saw this on another topic somewhere).

In the end, I'm starting to think that the problem is not the engine itself (or the cat) and more of a rear-axle issue.