Brand: Keep all four tires the same size, please

  • Article by: PAUL BRAND
  • Updated: August 22, 2014 - 10:38 PM

Q: My co-worker had a flat left front tire on her 2002 Jeep Liberty. She had no spare and the service station didn’t have her size. They sold her a bigger tire than the other three. I told her she would have handling issues, mileage would be affected and there would be stress on the front end, differential and transmission. There’s an awkward tilt of the Jeep toward the passenger side. I told her she would be at high risk for rollover. Am I right?

A: You win the Good Samaritan award! Assuming her Jeep is 4WD, mounting a tire of significantly larger diameter is absolutely wrong. The difference in rolling diameter will generate considerable stress on driveshafts, differentials and the transfer case.

I don’t think the “awkward tilt” of the vehicle would significantly increase the chance of a single-car rollover but it certainly won’t help vehicle stability. The binding of the drivetrain may cause a reduction in fuel mileage, but the larger concern is a potential failure in the drivetrain.

You did the right thing; now make sure she gets that tire replaced with one that is the same size and diameter as the other three. I’d be inclined to revisit the issue with the service station — they certainly should have known better.

Q: I hope you can help with my 1999 Durango 5.9 SLT with 115K miles. It runs fine but at inconsistent times — winter or summer, newly started or running a while, under load or no load or even parked — the engine begins to misfire and loses 80 to 90 percent of its power. No gauges or warning lights indicate a problem before or after the event.

I’ve been able to pull over, turn the engine off then restart it again and it runs fine just like the problem never occurred. In the last nine months the frequency of this has increased and a couple weeks ago I experienced a new event. While pulling a 2-ton trailer and slowing down for a stop sign, the engine completely died in a split-second. I immediately noticed the odometer did not show the typical miles numbers but did show jumbled lines and dashes. It did not start right up — it took three tries of cranking it for an extended period and then it started and ran fine.

I have repeatedly taken it in to a reputable mechanic and the local Dodge dealer for inspections and diagnostics that turn up nothing. Might you have some guidance for me?

A: Could Christine have morphed into a Durango? Intermittent issues can be, and often are, difficult to pinpoint even with modern on-board diagnostics built into the vehicle. Since there appear to be no DTC fault codes stored in the computer, I’d initially focus on potential mechanical causes such as a clogged/restricted catalytic converter or exhaust system. Exhaust back pressure can build until it literally chokes the engine. When the engine stalls, the back pressure is released and the engine may well restart and run fine again — for a while. A simple exhaust back pressure test with the pressure gauge screwed into the oxygen sensor port might confirm this — there should be less than roughly 2 psi of back pressure in the exhaust.

With the age of the vehicle, make sure the coil and ignition wires are in good shape and not generating any crossfire or grounding under load.

I can’t explain the bizarre odometer display, but it may indicate some kind of electrical anomaly that caused the stall. The best bet for pinpointing something like this is to plug a data recorder (co-pilot) into the diagnostic link and drive the vehicle until another event occurs. Lock the event data into the recorder and have the shop or dealership download the data — it allows them to monitor what happened in real time, hopefully pinpointing the culprit. Good luck.

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