Most likely culprit?
 

86 tpi mostly stock alum heads

Ok Last year I had an issue with detonation, so I changed a few things and it went away when I got the car out this year. So I figured something I did must of resolved it. Now it's back and it's steadily getting worse just like last year! I run premium and it started at mid-tank of the same fill up so I know it's not bad gas, plus it didn't do it at all with the year old gas(had fuel treatment) when I got it running, that was a few tanks ago. Just like someone flipped a switch it came back and is getting worse, so I am wondering if it's something like a sensor on it's way out. I got another coil & MAF to try, but figured it might be something else since I am no EFI guru.

Things I've done to the car before it started the 1st time
-all new/rebuilt ignition
-correct plugs gapped at .035
-new valve seals
-cleaned MAF
-cleaned TB and IAC
-set fuel psi to 44
-fan lowered(got a mild custom chip)
-new air & fuel filter
-removed egr(stfu)

Things I did this year
-new chassis & batt ground with other upgraded grounds.
-removed all cats
-wrapped exhaust to lower underhood temps
-new O2
-new plugs


Things I've done since it came back that helped some
-reset base timing to 6
-changed to colder plugs still gapped at .035 (old plugs looked good)


Other than parking it for another year and seeing if it goes away idk. The check engine light does come on for a few secs once in awhile, but it has always done that. I know I need to check for codes or w/e but the light doesn't stay on and is random. Also it just started as the weather cooled, but I'm not sure that's related now. Car was running great and now it's a detonation machine if you look at it funny.

You indicated you have a custom mild chip. Was this to turn the fans on earlier. Did you change the cam?

What happened to your stock chip? Since it does not look like you changed the cam a new chip/tune would not be required. I would be looking at you "new chip". Assuming you still have the old chip, swap it out to see if this makes a difference.

I know you said the check engine light only comes on once in a while. Check for codes....this could tell you something important about what is happening.

Detonation is a result of running LEAN. I would suggest hooking up the fuel gauge onto the rail and tape the gauge to your windshield. Run the car around the block or farther and watch the gauge to see where your pressure is sitting. It should be up around 36 PSI. If your running much less than 36, then look to the fuel pump and fuel pressure regulator. How old are the injectors?

The above will get you started exploring the problem.

I don't think it's the chip or it'd have always done it. Already checked the fuel pump, but thx for the suggestion. Injectors were replaced before I got the car, but I will test them.

edit: and no codes just checked

edit2: most injectors test 16.5, two 16.4 and one 16.7 Also replaced a vac cap that looked a bit cracked but I couldn't suck any air thru it. Bought an IAC since it was sometimes idle a bit high and I'll replace that later today. Read that when your temp is low always on a hot start(below 195) it won't test the knock sensor and throw a code so I may replace it. I'll pull the ESC module for my parts TA and swap it just for kicks as well.

Vac cap made no diff. Just got code 44 & 45, read it could be maf or the two relays for the maf.

Been also having a long start always and was reading about how it may be a leaking 9th injector or even the ECM. I have another ecm so I'll swap that when I get up.

FYI the fuel psi when driving is steady about 34 and goes to the set 44 at wot

Basically it could be tons of stuff

Update kinda.

Well I replaced the IAC, deleted the cold start, found one small vac leak, ported the intake & changed runners, did some heat mods on the intake and some or something in there helped some. The runners were full of dry carbon(I already replace the carboned up plenum with one I ported long ago)and when removed you could see the base was carbon & oil sludge, so I pulled that and clean/ported. Cooler cleaner intake should help but I couldn't clean the heads too good and I didn't want to remove them because I'm gonna build an engine for it later. I also reset(raised) the timing back up so I know it helped.

Then today I swapped the new coil with one from my C3 after reading about weak coils sometimes causing det and that helped more, but it still does some when nice and hot. I'd say it's down less than when I got it out this year.

edit: no codes so far since I put it back together

You changed and tested a lot of things, but there is one thing I didn't see. Did you ever check the Knock sensor? it's quite a easy test, get a timing light look at the timing mark and rev. it and see if jumps. Than raise idle let the timing mark settle out take a long extension place it of one of the Right side head bolts between cylinders #6/#4 give a tap with a hammer see if the timing jumps with the taps.

Try fresh gas with 93 or at least 91 octane. 'Try decreasing timing to 4degrees. One thing is for sure, if you don't stop the detonation, you'll be rebuilding sooner than later. Did the aftermarket chip increase the timing curve over stock?

It was so bad it wouldn't correct when I started this thread unless I let off and why I changed my T-stat to a 180 from 160 also(forgot to mention that is the changes) Now it corrects if I get det part throttle, so I'm pretty sure it's working somewhat.

In my 2nd post I mention the temp knock sensor idea and I know it was working before it got bad. Here I asked about some of the operating temps and I can't seem to find consistent numbers form my searches. https://www.corvetteforum.com/forums...loop-temp.html

I almost replaced it when the coolant was drained to be sure. Still I'll try a hammer test to be sure, if I can get a helper LOL Thx for the info.

Last year it started up before I parked it and I got a milder almost stock tune this winter and now the tuner is not responsive. So I don't really think it's the tune because it was fine then it got worse with the two diff tunes over time. For all I know it'll get progressively worse form this point.

I'm not 100% ruling out fuel and will deff get it from someplace diff. I'm still on that tank it got bad and has progressively gotten better with changes. Tonight when I go out I should be close to half tank and I had planned to top it off with some premium from another station.

6deg was better but still worse than it is now with the changes and it's at 10deg

stock tune, new gm knock sensor, and reconnect a good gm egr valve will fix your problem. good luck

Without changing heads w/larger chambers next fix would be to retard the cam. Since its a GEN I block there are all kinds of timing sets and bushings u can use. I wouldnt waste time and effort and just retard the cam 4 degrees to start with. This shuts the intk vlv later and reduces dynamic compression. Best of it is u can leave the intake on and cam in the block. just replace the timing set - move the crank/not the cam for the timing change. Youll lose some peak torque but alternatives are more expensive.

Good luck.

Well right before I got gas the rpm went up 100 in gear/idle and it started to run better. Just like someone flipped a switch. Anyways I put 8.7 gallons in from a brand new station and it ran just like before it started acting up. I'm gonna check for vac a leak again today. Really bugs me how it changed on it's own while driving.

Still got it....

As I said in the previous post right before I got fuel it got some better then I swapped my coil from the c3 and it seemed to happen less. I've also increased the rpm and gave it some water through the booster vac line a few times to try and break up carbon if it's the cause, but not enough to hydro lock it.

Car is acting up again some and det is worse but not real bad like it was. I've topped off at a brand new busy gas station two times for a total of 19+ gallons. Also now I have CODE 33!!! and 44 but no more 45. 33 is a real pita form what I'm reading or I could get lucky and it's just the burn off relay(86 is four pin).

My c3(carb not efi) had some det this year too I noticed and I gave it a brand new set of the same plug wires so now I kinda suspect them for part of it maybe. But since it has gotten worse and then better on the same fuel and now with the dreaded code 33 I'm thinking it could be a hard part like a sensor that is failing.

Figured I'd post this link for those who still confuse detonation with pre-ignition
http://www.contactmagazine.com/Issue...ineBasics.html

I'm about to park the car so it may have to wait until spring to find the true or final cause.

So what was the time frame between your getting the custom tune and when the problem appeared. Is it all the time or random. Where did you get the chip? Was it a dyno tune with the sniffer in the tail pipe or mail order off the shelf some place.

I would really expect the chip to be the source of your problem. When I first got my 85 and didn't know the Forum existed, I bought a Hypertech chip to get another 50 horses. Over the next 2-3 years I had problems with ping...or as you say detonation. It would come and go. Didn't seen to be any logic to it. Changed grades of fuel, tried octane boost and finally played with timing. I had a custom dyno tune on the car and my tuner discovered that as the throttle position increased that the chip was increasing the timing and at the same time removing fuel! His tune solved that issue, MPG increased and a small increase in power on basically a stock engine at that time.

Bought the car winter 2013 and parked it, notice it had some knock/det on the drive home. I did a complete tune up still had some knock/det. Spring 2014 put a new tune in and it seemed good but det got worse the more I drove it(very rare knock). This spring I put another tune in by the same online tuner and no knock since, but det still. He did add the hookup so I can datalog, but I don't have a laptop even :) That and a gopro are on my winter buy list. Gonna need a latptop for the MSD & AFR on my c3 anyway.

I've changed a lot of stuff on the car period, because whether I get it figured out or not it's gonna be a drag car even if I have to go to carb or put a new engine in. I've put around 700miles on it since I bought it LOL. I don't drive it a lot because I'm always tearing it apart to make it into a faster car or working on one of my other cars.

I know a custom tune is ideal, but I've always herd the MAF cars are not so picky with changes. And I've not made a lot of radical engine changes IE cam, heads, displacement, compression. It's pretty much had the problem since I got the car and I'm trying all the stuff I can myself before I get a dyno tune. Because the car is not done and I'm not gonna have it re-tuned every other day. I'm not 100% sure if I'll stick with EFI and don't wanna invest a fortune into tunes, gear to tune it myself or some aftermarket system until I decide to stick with efi.

edit: Oh and the tuner knows of my det problem I gave him every detail of the car and this was suppose to be a real mild tune this year.

Well changed both maf relays and idle went up some, but then it started to idle rough after it got warmed up. Been doing that only recently, like past few times I drove the car and when I got the check engine light code 33. I know the new burn off relay is working, I can hear it clicking when I shut the car off.

The I changed the ecm with the reman I bought for my TA when I did the 165 convert and the rough idle hasn't come back so far. The exhaust smells a bit better, not that it was bad really(compared to rich carbs I've had to tune) just smells diff.

For each swap I unhooked the batt and the idle remains about 100rpm higher. I'll drive it when it's dry tomorrow, my drag radials are pretty worn. Starting to think it's more a sum of problems than just one.

Test drove the car and it's better, but still there some. After the drive idle went up a bit more. I still can't find any vac leaks.

Have you changed the plugs?

I had spark knock noise when I ran certain plugs in my 86.

Worst were the torque master spark plugs. Most of my problems with it were at very light throttle with it clearing up when I pushed it further. Basically the plugs were igniting the fuel without actually being "sparked" by the distributor.

Also a bad cap can cause that effect. I had the same thing in a vortec motor in a truck I had. Now it was a real pain to straighten out because I kept changing caps. It ran ok for a bit after messing with it aka changing the cap then was back acting up. What it turned out to be was the dust in the bottom of the distributor. I cleaned all of that out and no more pre detonation. All I can figure is the stuff in the bottom of the distributor was getting back up onto the new cap after I changed it each time.

All long shots but it seems like your in need of the not so common possibilities at this point :)

One of the 1st things done before I started this thread was change from autolite 3924 to a NGK v style colder plug. I don't rem part number, but it's one colder than the ngk they list as a direct app. It got a bit better with the plugs, FYI both were gapped .035 Been considering another step colder, but the engine compression is stock and the heads never off far as I know.

Dist was cleaned and completely taken apart and rebuilt when I got the car. But the rotor would stir up dust somewhat as it spun if there was some. I use a decent cap & rotor I think, with a low res bushing and the center of the rotor pulled up so it makes contact with the bushing. I also use some dielectric grease under that spring tong in the center, MSD recommends it with their rotors so I do it on all now. I use electric contact spray on the cap anytime it's off and wipe down the rotor, being sure to never touch either with my hands. I do a few other things to lower chance of misfire, but I'm not gonna go on and on. It was off just recently with the intake porting and looked to be in good shape.

Ok one other trick/fix for vortec motors which are horrible about carbon buildup that might help.

If you had carbon chunks around the ports on the heads when you had the intake off then there is a very good chance its in the heads themselves which increases your compression ratio thus will cause the problems you having.

Some use seafoam in the gas or intake to clean it out but I personally haven't tried that. What I do know works flat out is gm's cleaner. You have to get it thru the dealership I think. It will remove ALL carbon from the cylinders. That's the good news. The bad news is it doesn't always "flake" it off or slowly dissolve it. Sometimes the chunks just let go and some people have had valves bent from it. I haven't had that problem but I'm just telling you it has known risks. But it WILL remove all the carbon.

Most people use the above method. I use a slightly different setup for doing it but it involves tools you aren't going to have so I listed that method.