Loss of HP
Thread Starter
4th Gear
Joined: Jan 2017
Posts: 4
Likes: 0
Loss of HP
Hello. I recently got an 85 corvette and it runs great but I feel some power loss when accelerating. During idle sometimes it would make a rattling noise. And when I accelerate the noise goes away but comes back and I feel the engine losing power. I recently changed the pvc and the maf sensor. Need help. Keep losing to hondas👎
Team Owner
Joined: Jul 2004
Posts: 28,357
Likes: 3,253
From: Hartford WI
I honestly don't place any faith in the Seat Of the Pants dyno. It is really hard to know what is going on. If you suspect anything, you probably need to check and see if there is some issue or not. You need to make sure the fuel pressure is right and there has to be no air leaks and everything is functioning as it should be. Also, that noise could be something breaking or not working right and it can cause the ECM to retard timing which cuts power. Probably have to get someone to scan it and see what the knock counts are for a regular motor and what your's is. How are the plugs, wires, coil, injectors?
Thread Starter
4th Gear
Joined: Jan 2017
Posts: 4
Likes: 0
Plugs and wires are new also but injectors and coil seem to be working fine other than the rattling noise. Before I got it, The car sat for ten years and has only been driven twice. When the rattling starts I can feel the power loss and it's pretty noticeable because when in full throttle it would launch pretty fast and at a high rpm the rattling would start and the RPMs stay the same. Then when the noise goes away it regains the hp loss. I can hear the noise over my loud exhaust. And it's comes from around the intake area and when I place my hand on the intake manifold I can feel the rattling.
Team Owner
Joined: Jul 2004
Posts: 28,357
Likes: 3,253
From: Hartford WI
Plugs and wires are new also but injectors and coil seem to be working fine other than the rattling noise. Before I got it, The car sat for ten years and has only been driven twice. When the rattling starts I can feel the power loss and it's pretty noticeable because when in full throttle it would launch pretty fast and at a high rpm the rattling would start and the RPMs stay the same. Then when the noise goes away it regains the hp loss. I can hear the noise over my loud exhaust. And it's comes from around the intake area and when I place my hand on the intake manifold I can feel the rattling.
I would scan it and see if there is excessive knocking since that can retard timing and power.
Team Owner
Joined: Jul 2004
Posts: 28,357
Likes: 3,253
From: Hartford WI
Drifting
Joined: Mar 2013
Posts: 1,952
Likes: 239
From: Venice Cali.
You also need to start at #1 set base timing. If it is off it will knock when it should not be.
Then do a idle adjust. They can be found somewhere here. Mabey a bad knock sensor also. Good luck
Then do a idle adjust. They can be found somewhere here. Mabey a bad knock sensor also. Good luck
Last edited by xrav22; Jan 24, 2017 at 12:49 AM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
Team Owner
Joined: Jul 2004
Posts: 28,357
Likes: 3,253
From: Hartford WI
Better hold off on that one since we aren't having idle issues. Besides, the procedure is kinda stupid since it only works IF everything is within spec. IOW, it assumes much, knows little. If he has to set it, I would clean the TB (remove IAC, IAC housing and top plate, flush all passages), fresh gaskets and set with scanner.
Hard to say but if it doesn't give any input, the ECM will note it is bad so that might not say much. I would say a scanner will help.
Thread Starter
4th Gear
Joined: Jan 2017
Posts: 4
Likes: 0
well I changed the maf sensor because it bogged down after 2k rpms and wouldn't move faster. After I changed it, everything was running strong until a couple days passed and I started hearing the noise. I've done a full tune up and nothing has changed but I noticed that cylinders 2 and 6 have a steel sleeve for the spark plug. I dont know if that affected anything because they were a bit loose. But now it revvs up and down during idle
Team Owner
Joined: Jul 2004
Posts: 28,357
Likes: 3,253
From: Hartford WI
well I changed the maf sensor because it bogged down after 2k rpms and wouldn't move faster. After I changed it, everything was running strong until a couple days passed and I started hearing the noise. I've done a full tune up and nothing has changed but I noticed that cylinders 2 and 6 have a steel sleeve for the spark plug. I dont know if that affected anything because they were a bit loose. But now it revvs up and down during idle
Probably someone stripped the threads and helicore was done. Check compression and fuel pressure especially at WOT
Thread Starter
4th Gear
Joined: Jan 2017
Posts: 4
Likes: 0
Well I finally got everything checked out and the distributor cap might be broken because when I put my hand next to it, it shocked the crap out of my hand. Compression is good. Got the injectors checked out by a mechanics shop. They told me the rattling may have just been the spark jumping to something metal.
Last edited by Alexis Contreras; Feb 2, 2017 at 06:59 PM.
Drifting
Joined: Mar 2013
Posts: 1,952
Likes: 239
From: Venice Cali.
Be careful that's high voltage. I have new wires and they need to be manipulated because I saw a spark from distributor to wire between 1 and 2 then I read to make sure they are separated nicely. No more sparks.
Team Owner
Joined: Jul 2004
Posts: 28,357
Likes: 3,253
From: Hartford WI