Hi. Two separate topics. Maybe I should have started two threads? Anyway, first, I just bought a 1999 S70 with the Turbo and AWD. Both of the front tires look almost new, but the rear tires look to have only about 60% of the tread left. Same make, size, etc. Is this a problem for the Volvo AWD system? Second, at -3 degrees Fahrenheit, the engine turned over slowly, but it did start. The battery is 3 years old. Is this "normal"? Also after I have been driving the car for a while, and it is well into the operating temp range, if I shut it off and go into a store, when I come back out, half the time it turns over fine, but takes a while to fire. The other half of the time, starts good. Any thoughts? Coil packs and spark plugs were replaced 13,000 miles ago (4 years). 155,000 miles on car. Thanks.

 

AWD cars for some reason require all 4 tires to be very close to the same diameter. Hence why they tell you to replace all 4 tires at the same time when needed to avoid costly damage to the AWD system. If it was me, I would put new rubber on all 4 corners as soon as possible. Any car I have ever had turns over slow at -3, the oil is pretty thick at that temperature. You could change to a thinner weight oil to help starting, but it is pretty normal to turn over slow considering the temp. As for the starting issue when hot. I would do a full tune up.....all new fluids and filters, new Volvo OEM or NGK plugs (Volvos are finicky about plugs for some reason) I would include a new coolant temperature sensor as part of the tune up.....really easy to change as it is right below the thermostat. Good luck.

 

for the most part, Volvos are FWD until the car detects slippage then engages the center differential. Its true you do want to keep the tires at the same circumference to avoid the fronts/rears turning at different rates so as to not put any stress on the center differential. You do have some options - if the fronts are new, replace the rears with the same size/make. You can measure tread depth to determine wear and the percent difference on your current tires. You can also check out tire shaving services which will take two of the tires and grind them down to match the other two. I believe TireRack actually provides such a service - maybe worth a call.

As to the slow cranking, first thing I'd do is clean up the battery terminals and inspect the cables, particularly the one to the starter. If you find any corrosion in the cables (ie bubbling/swelling, melted or odd looking insulation), its time to replace the cable. Assuming your ignition is in good shape, check plug gaps. Don't assume new out of the box are correctly gapped for your model. +1 on going back to OEM plugs and checking the temp sensor. Also, it may be your battery is a bit undersized for sub zero F. Most cars will spin a bit slower at that temp but you should see what battery is in the car and consider an upgrade to get some additional CCAs.

 

Thanks!

 

The ECT sensor was replaced about 60,000 miles/10 years ago (car now has 155,000). It also looks like the last the engine air filter was replaced was about 65,000 miles ago, according to the invoices I was given. It was pretty grey looking!

 

As mentioned, the Haldex style 4 wheel drive will start to apply power when the front is turning faster than the rear. Therefore, in theory, having the smaller diameter tires on the back should work great for every situation, even better than having all tires the same size, but that's theory. You can believe it or not.

On the difficulty starting, do you want to troubleshoot it? If you do, then you have to catch it not starting and look at some things. With a scantool, you can tell what the engine thinks the temperature is, so that's easy. Fuel pressure is not so easy. If it was my car, I might be tempted to blame the fuel pressure for taking a little time to come up to normal. A tune up is a good idea. I don't like to bother with the fuel pump unless I know it needs work. If it is the fuel pump, then it's likely the problem gets worse and worse, and eventually you're stranded, and then it becomes easy to troubleshoot, if you want to troubleshoot. A lot of people don't like to.

 

So, if my current front tires are almost new, I could buy two of the exact same tires new, put them on the front, and my current front tires on the rear, and I should be ok. I assume the front tires will wear considerably faster than the rear, since they are getting the engine torque and steering (most of my experience is with 4x4 pickups).

I put a new air filter in, and that seems to have taken care of the hard starting when warm issue. The air filter was really dirty. Cold start needs less air, hot start more?

Thanks.

 

cold air is denser but cold starts also have a richer mix to help out (like a choke which cuts back air for a carb...). If you had a dirty air filter, you'd be running too rich on a warm day - which can in a sense, blow out the spark...