98 Miss
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98 Miss
Hi Guys,
Background: 98 coupe w/74000 miles. No internal mods, but the typical bolt-on's (intake, headers, corsa exhaust, etc.). Car has had mods for several thousand miles without issue.
Symptoms: No codes being thrown. Car struggles to stay running (but does), especially after coming off the highway at sustained cruising speed in the 70-75 mph range. Runs PERFECTLY fine under heavy throttle or even WOT. Under light acceleration, it feel like there is a slight miss. Mechanic says that scanner is showing big timing adjustments being made after coming off the highway.
Constructive thoughts/suggestions more than welcome. Vacuum leak?
Andy
Background: 98 coupe w/74000 miles. No internal mods, but the typical bolt-on's (intake, headers, corsa exhaust, etc.). Car has had mods for several thousand miles without issue.
Symptoms: No codes being thrown. Car struggles to stay running (but does), especially after coming off the highway at sustained cruising speed in the 70-75 mph range. Runs PERFECTLY fine under heavy throttle or even WOT. Under light acceleration, it feel like there is a slight miss. Mechanic says that scanner is showing big timing adjustments being made after coming off the highway.
Constructive thoughts/suggestions more than welcome. Vacuum leak?
Andy
Racer
Joined: Feb 2007
Posts: 403
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From: Glen Allen VA
Hope this helps.
Mine is a 98 C5 coupe MN6 with 94k miles and had the same problem this summer but it was missing at lower rpms and high gear. Long story short it turned out to be a lose spark plug wire. I must have bumped it while cleaning the engine bay, the neat-freak that I am. I ended up replacing all of the spark plugs and got GM performance spark plug wires while at it and shes been running smooth since then.
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Joined: Dec 1999
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Sounds exactly like the symptoms of a vacuum leak or your getting UN METERED air into the engine.
Check ALL (and I mean every) vacuum line/s. Your PCV system is well known to deteroriate and leak. It runs arounf the back of the engine behind the intake manifold:
You can see the hoses and rubber coupler in this pic:
Carefully check the ducting between the MAF and the TB. It MUST be 100% SEALED AND CAN NOT LEAK AIR.
Have you ever cleaned the MAF and TB????
If not, I would do that also.. Purchase some MAF cleaner and spray it clean.
Let us know how you make out.
BC
Check ALL (and I mean every) vacuum line/s. Your PCV system is well known to deteroriate and leak. It runs arounf the back of the engine behind the intake manifold:
You can see the hoses and rubber coupler in this pic:
Carefully check the ducting between the MAF and the TB. It MUST be 100% SEALED AND CAN NOT LEAK AIR.
Have you ever cleaned the MAF and TB????
If not, I would do that also.. Purchase some MAF cleaner and spray it clean.
Let us know how you make out.
BC
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Joined: May 2011
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98 Miss
Thanks for the comments guys.
Turned out to be fuel related. Added some HEET to the fuel, ran it 20-30 miles and issue is completely gone.
However, your suggestions re MAF and TB cleaning are well founded, appreciated, and will be acted on.
Turned out to be fuel related. Added some HEET to the fuel, ran it 20-30 miles and issue is completely gone.
However, your suggestions re MAF and TB cleaning are well founded, appreciated, and will be acted on.
