Quick answer. A 2018 Mustang GT in Beaumont was idling rough, stumbling while driving and reading 16 to 18 on the air fuel ratio. On August 30, 2026 it was quoted 2 hours of diagnosis at $150 an hour, plus a set of 8 NGK iridium plugs at $25 each. The job record does not say what the cause turned out to be.
Key Points
- The car was a 2018 Ford Mustang GT in Beaumont.
- It idled rough and stumbled while driving.
- The owner saw air fuel ratio readings of 16 to 18. That is lean.
- The quote had 2 hours of diagnosis at $150 and 8 NGK iridium plugs at $25 each.
- The job record does not say what the cause was, so we do not claim one.
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What the Owner Saw
The owner gave us real numbers, and that helps. Rough idle. A stumble on the road. An air fuel ratio between 16 and 18.
Gas engines like to run near 14.7 parts air to 1 part fuel at cruise. At 16 to 18 the engine is getting too much air or too little fuel. That is called lean.
Lean makes a V8 shake at idle and stumble under load. Left alone, it can run hot inside the cylinders.
Where the Testing Starts
Our quote put it plain. A lean reading that high with a stumble usually points to a vacuum leak, an intake gasket or an injector problem.
Usually is not proof. So the plan was to hook up a scanner and read live data first. Fuel trims tell you if the computer is adding fuel to make up for extra air.
No parts were ordered for the cause until it was found. The quote said we would call with the fix and price before doing any repair.
2018 Ford Mustang GT, Beaumont, August 30, 2026
Came in with: Rough idle, stumbling while driving, air fuel ratio 16 to 18.
Likely suspects in the quote: Vacuum leak, intake gasket or an injector.
Plan: Scan tool and live data to confirm before any repair.
On the quote: Diagnosis, 2 hours at $150. Eight NGK iridium plugs at $25 each.
The Plugs on the Ticket
The ticket also lists 8 NGK iridium plugs, part LTR6IX-11, at $25 each.
The record does not say the plugs were the cause, and we will not say it for it. Plugs are on the ticket. That is all we know.
The diagnosis hours come off the repair if the owner has the work done with us.
Frequently Asked Questions
What does an air fuel ratio of 16 to 18 mean?
It means the engine is running lean. It is getting too much air or not enough fuel.
What causes a lean rough idle on a Mustang GT?
Our quote named a vacuum leak, an intake gasket or an injector as the usual suspects. Live data confirms which one.
How much was the diagnosis?
The quote had 2 hours of diagnosis at $150 an hour.
Does the diagnosis cost come off the repair?
Yes. On this quote the diagnosis came off the repair if the work was done with us.
Supercanic tip: If you have a wideband gauge, write down the numbers at idle and on the road. Real numbers aim the testing. Call us at (951) 904-2477.
V8 Running Rough?
We read the live data first, then tell you the fix and the price.
