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Discussion Starter · #1 ·
Hello all,

Jump-started my 84 MKII 5M-GE 5-speed manual on Saturday and the engine ran very rough and at low RPM. I had to keep my foot on the gas while idling in neutral to keep it running. When I got out to close the hood, the engine stalled. Couldn't get it started again. Haven't driven it much in 2021.

I've read on this forum that the EGR valve might relate to rough running? Is that correct? Any other tips would be super.

The fuel pump was replaced a few years ago and the fuel pump relay was replaced last year. Head gasket replaced in 2004. 330K KM on the original engine, though odometer and speedo have been off ever since the transmission was replaced about 10 years ago at around 300K KM.

Best,

SJ
 

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I had to keep my foot on the gas while idling in neutral to keep it running.
When its cold or until the ideal operating conditions are achieved, it actually needs to be rich, or a little extra fuel until it warms up, maybe as little as only nine units of air per every unit of fuel. The computer is only responding to the air that's being metered through the air flow meter and adding some fuel based on the temperature sensor to make the mixture properly rich approaching stoichiometric ratio as it warms up. Sounds like maybe a vacuum leak after the AFM since you were giving it both more fuel and air to keep it running. When the throttle is closed and the engine at idle, a small vacuum leak, like just a loose hose clamp, can be a larger percentage of the total volume of air coming in, and thus significantly alter the air-fuel ratio beyond the range where it will run. When you open the throttle, the computer reads the throttle position sensor, adds fuel, that small vacuum leak becomes a very tiny percentage of the total volume of air and thus has far less impact on the air fuel ratio.

Always diagnose in order starting with the cheapest thing to fix first. The cheapest thing to fix is the base ignition timing so check that first and make sure the distributor is adjusted properly. Vacuum leaks are usually the next cheapest thing to fix (tho not always the easiest thing to find - you can search youtube for methods if you don't find anything obvious like a broken or cracked tube - at this age, vacuum hoses and intake piping can be brittle). Check those thoroughly, then if you don't find a leak, get back to the forum and someone will suggest some sensor and fuel system checks next. Fingers crossed that its something simple.
 
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Discussion Starter · #4 ·
Thanks for the replies, pdupler and dannoxyz. Good info for the future. Turned out in this case to be the alternator. Running well now and got it stored (late) for winter, though I still need to jump-start it since the five-year old battery is toast also. I've read that a bad battery can't harm an alternator but a bad alternator can harm a battery. Thoughts? Best, SJ
 

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IMO, a bad battery can hurt the alternator ,causing it to charge like crazy to try to keep up! It's gonna get hot!
An alternator can be bad to under charge OR overcharge a battery.
A battery tender is a really good idea. One or 2 amps, automatic one. That also eases the load on the alternator on start up.. ;)
Winter storage? Check for .....🐭🐭🐭 !!!
There are ways to reduce the invasion.
 
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Measure actual battery voltage:

1. battery voltage everything OFF

2. battery voltage while cranking

3. battery voltage at idle

4. battery voltage at 3000rpms


These numbers will give insight as to what may be wrong for further examination.
 
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