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Old 05-05-2015, 06:18 PM   #1
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New knock in an old-ish Mustang, could use advice


A friend of mine donated to me a 2004 Mustang, 3.9L (auto) with 137K miles, via my church to replace my dead Civic (265K miles and one seriously warped #1 cylinder later...). It's been great so far, but recently a couple things started worrying me.
First, I noticed the temperature spike after a fairly short drive, and discovered I had very little coolant left. Filled it with water (remembered the vent too) and checked it, and found the water pump leaking around the pulley seal, so I replaced that. Relatively easy fix, thank you Ford!
Only that wasn't the problem...around the same time, I noticed a barely noticeable loss of power, like a cylinder missing maybe, and very rare (not cyclical). On that drive home (to park it 'til I fixed the water pump), there was a short bump up in the temperature - not all the way to the red, maybe to 3/4's up the gauge, then it went away as I drove a little farther (looking to find a place to pull over if it became necessary). After that though, even though everything (oil pressure, temperature, service eng light) was normal according to the gauges, the engine started to sound like it was pinging. It was only doing it under load, not at idle, and after I parked it I revved the engine a couple times and nothing. It didn't sound like lifter knock, more like detonation or excessive timing advance. I had to use it once more, and it was fine while the morning was cool, but for the return trip in the afternoon heat, it started making the sound about halfway home (7 miles or so each way). If I minimized my throttle and coasted when I could, it sounded normal. It was worst in high gear (i.e. overdrive) when the engine was running at lower RPMs (1400-ish) if I wasn't coasting. When I got home again, I put the car in neutral and revved once and the engine sounded perfectly fine.

I'm not completely clueless, but my '98 Civic was the newest vehicle I've worked on, and a '91 Geo was the most depth (tore down the engine and had it machined). Is there a timing-related sensor that could have gone bad, or something that's not adjusting fuel flow through the injectors for the increased throttle? If it was constant I'd think it could be the lifters, but it's not - it seems to start once the day starts to heat up. I don't know the details and "issues" for this engine either, so I figured I'd better turn to the "almost pros" or who knows, actual pros too...

Anyway, sorry my first post is a "HELP" post, I do plan on contributing and not just coming to beg for help. Right now, though, I don't want to think it's no big deal and find out I'm about to throw a rod. Been there with the Geo, done that, don't want to do it thanks in advance for the help!
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Old 05-05-2015, 10:18 PM   #2
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Location: Salinas
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Even though the Check Engine Light may not be on, you should take it to an auto parts store and have them scan for any codes. And write down the number of any codes!
It will make it a lot easier to diagnose this problem...
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