Passat Overheated once, Repaired, Now engine misfiring

My 2003 Passat v6 with 56000 miles overheated last week. As soon as the “STOP” message came on the dash, I turned off the engine, and consulted the user manual. I discovered the coolant level was very low, and a tiny bit of steam had come from under the hood.

I filled the coolat system with distilled water, and the car drove fine and did not overheat again. Next morning, the mechanic diagnosed a leaking heater core and lot of crud in the cooling system. I had them replace the heater core and flush out the cooling system, and paid.

The mechanic then reported a noise coming from under the timing hood cover. I drove the car and it ran extremely rough, and the check-engine light was flashing. I went to autozone and they hooked up a diagnostic handheld thing and told me that 3 of the cylinders were misfiring.

What could have caused this? The engine overheat happened once and I turned off the car within a few seconds. Right after the cooling repair was done, the engine cylinders started misfiring.

why didn’t your mechanic diagnose the problem before he/she let the car leave?

“The mechanic then reported a noise coming from under the timing hood cover.” I’m assuming you mean the timing belt cover. Am I correct that the noise was not there prior to the time the mechanic worked on the car? Is there a constant noise, or did the noise happen once?

It reads like you paid the bill and then the mechanic told you about the noise. Is that the sequence of events?

If the CEL was on, why didn’t the mechanic check for codes. I don’t understand why you had to go to AutoZone. Something doesn’t add up.

Is this a mechanic you’ve dealt with in the past, someone who’s familiar with your car?

I know the shop from a long time ago when they did a good job on a very old VW. I went to them after 10 years this time (same owner).

I was a bit upset that there was no noise and no cylinders missing before the repair, but showed up after the repair. So I wanted to take the car and get a quick second opinion.

You are correct that the noise wasn’t there before the mechanic worked on it.

It’s likely that your timing belt slipped. The valves in three cylinders may be bent.

Could this happen during the cooling system repairs, or due to the brief overheat? And is that an expensive fix/adjustment/replacement?

Change your thinking from “the problem started after the repair” to “the problem started after the engine overheated”. You’ll find the solution more readily.

My suspicion is that the engine overheated and the coolant got blown out due to a blown headgasket, the head warped due to the overheating, and that head gasket breeches are what the mechanic is hearing ticking away. It’s probably also allowing coolant to be drawn into the cylinders, preventing the plugs from firing and causing the CEL light to flash.

Start with a compression leakdown test. That’s a test wherein compressed air is pumped into each cylinder with e gage attached to the air supply line. The line is closed off and the pressure watched to see if it’ll hold pressure. My bet is that three of the cylinders will not, probably all in the same bank (same side of the engine). My guess is that on that bank at this point the head gasket is blown and the cylinder head warped.

Post back with the test results.

Mountainbike has presented a very plausible scenario.
A leak-down test will tell the tale.

This is great advice, and gives me a lot of faith in the shop I went to. They took a similar problem scenario path and did a compression test (which failed in several cylinders) and then recommended I tow the car to the VW dealer.

The other thing the mechanic said is that the water pump impeller may be breaking up … and that may have caused the overheat, which caused the coolant to burn up, which may have caused the crack in the heater core …

Should find out tomorrow what the dealer says, and I’ll report back.

Thanks all you experts again.

It could have caused the overheat. Price of repair is heavy if it is true. I could be wrong and the next post is better than mine.

Thanks for posting back. So many abandon us.

I wish you the sincere best and look forward to reading the final results.

The final analysis:

  • The water pump was bad.
  • The busted heater core spewed enough coolant out into the timing belt etc, which caused it to slip.
  • Luckily, no bent valves.

Replaced: Water pump, timing best and tensioner, heater core.


  • my mechanic thinks the water pump caused heater core to blow out, dealer thnks that the heater core and the water pump are coincidental failures.

Again many thnx to all the experts on this forum.

Thanks for posting back. I’m glad I was wrong. Your actual problem was much less destructve than a bad head would have been.

Kudos to pleasedodgevan for guessing that you had a slipped timing belt.

I’m having trouble envisioning the bad water pump as stressing the heater core. The system pressure would be limited to roughly 16 PSI by the radiator cap, and if the coolant drained the heater core would be subject to no heat or pressure.

Nomatter, I’m just glad you got it fixed.

Thank’s for the good comment. Bonus answer.Sometimes when the overheat happens, the heater core can’t take the pressure. They aren’t as solid as they used to be when they were copper, and even then they still could pop. Everybody wins here.