Rough idle 99 gmc k1500 5.7

Asked by Jesse Oct 24, 2016 at 12:03 PM about the 1999 GMC Sierra

Question type: Maintenance & Repair

Truck idles very rough but once it's over 30 mph it
seems to drive just fine until stopping or slowing
below 30mph. I have replaces plugs twice, wires,
fuel oump, fuel filter, relay fuse, full distributor. Any

1 Answer


Could be a vacuum leak , but I suspect it's the MFI . (Multiparty fuel injection). This is a notoriously plagued system .I had it on my suburban as has just about anyone with the same.set up . It's called the (Spider injection ) .and it's a faulty design from day one .they make upgrades that will alleviate all of the problens associated with then. Rough idle , bad throttle response ,missong/skipping port all around performance. Now I'd check for a vacuum leak and make sure you've gapped spark plugs correctly etc. Just to be sure all other avenues are explored before looking into replacing and upgrading the MFI . It's about 100 bucks more than going back with the original set up's a world.of difference aND brings new life to your engine .Good luck

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