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Matt, Engineer
Category: GM
Satisfied Customers: 21871
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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2000 gmc jimmy starts hard but seems to run ok when running

Customer Question

2000 gmc jimmy starts hard but seems to run ok when running black shoot came out of exhaust
JA: How many miles are on the car? How has your mileage been lately?
Customer: my sons car around 145000
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: yes if possible
JA: Anything else you think the mechanic should know?
Customer: I thought it might be the fuel reg but the maniflod is different than the one I looked at
Submitted: 1 year ago.
Category: GM
Expert:  Matt replied 1 year ago.


This could be incorrect fuelling due to a faulty watertemp sensor. If this is the problem then the ECU will be leaving the car constantly ' on choke' and not reducing the fuelling as the engine warms - so unplug the sensor and clean the contacts on both sides with some switch / electrical cleaner (not WD40) and inspect the wiring for any signs of damage to the insulation especially where it runs over other components and through the bulkhead- if this doesn't help consider replacing the sensor as they're not expensive

Check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers