Detonation under acceleration

[K5owner]

I am kind of stumped on this one so I am throwing it out there to you guys and see if a fresh set of eyes and guide me. So some background first, I am the third owner of a 1984 K5 with a factory 350, 700r4 with a/c. The original owner was the City of Norco in California. This is a CA emissions vehicle with the CCC quadrajet and no vacuum advance on the distributor. It is all controlled by the computer and associated vacuum devices. It is a very original vehicle so I know this is how it left the factory. Okay now onto the problem. I recently had the carb rebuilt because I was getting the typical leak from the casting plugs and had some throttle shaft slop I wanted to get rid of. I get the carb back and it runs well until I get into the throttle then I get pinging or detonation. On light throttle and partial cruising it is all fine. Here is a list of what I have done so far:

new cooler autolite 24's (factory calls for 26's I believe)
new cap and rotor
new plug wires
new water pump
new radiator
rebuilt carb
adjusted timing to 6 degrees per underhood specs
fresh 89 octane fuel

Now I have read a faulty EGR valve can cause my issues as well as temps that are too high. The truck has always run at 210 no matter what. The previous owner lived in the mountains and put a warmer thermostat in it to bring up his heater temps for interior comfort so I know that isn't how it is supposed to be. Another thing is when I tested the EGR valve it will hold pressure but bleed off pressure slowly but noticable on my vacuum pump. I ordered a new (correct GM part number) EGR valve and a 192 degree thermostat to replace those two things.

Now mind you I never had any issue with this truck pinging BEFORE I had the carb rebuilt. In fact it had more initial timing (12 degrees) in it before I rebuild the carb, it ran fine other then being rich from the carb issues i described. So what do you guys think? Carb? EGR? Temp too high? I am kind of running out of ideas here. I have yet to touch anything on the carb since getting it back from the shop that rebuilt it in fear of chasing my tail. It ran good out of the box as they live test and tune the carbs prior to delivery to the customer. Is there any APT or secondaries I can richen on this carb or adjust? I need to get it running solidly as this will be the rig we take for thanksgiving camping here next week. Thanks guys I appreciate any help.

[Possmguts]

Interested in reading replys on this.. Im sort of having the same issue except mine is a 67 with a Holley. I have to put a bottle of STP octane boost in with fill ups to get rid of the pinging. Im at 10.5 degrees on the timing.

[rich weyand]

A bad MAP sensor can do that. If the computer doesn't know that you have your foot in it, the timing it produces will be too advanced.

[K5owner]

Okay. Anyone know where or have a picture where the MAP sensor is on an 84? Or could it be called the differential pressure switch?

[mattybilt79]

Sounds like its running lean or timing is not advancing under load. I would start by calling the carb builder they should have some sort of tech line to help you adjust it. You should also be able to put some timing back in it but advancing timing too far causes detonation and you dialed back and got it so I would start with the carb first. Those mid 80s carbs are a pain especially in communist California with smog required. Might be carbon build up in heads causing pinging considering it was a city truck cruised around town in traffic started and stopped a lot. You said p.o. lived in mountains and drove it in cold weather a lot too. I had an 84 k5 too when I lived in so cal when it came up due for smog it failed due to high hydrocarbon reading my guy said try egr so I pulled it off and sure enough packed full of soot build up. I put new egr on and dumped atf down the carb which is supposed to help clear carbon build up but is bad for cat converter. Ran it hard down the crosstown freeway to clear everything out took it back and passed smog with flying colors. So it may be simply the way your k5 is being driven causing the issue but still would start with the carb, might be something as simple as fuel pump going bad and carb not getting enough fuel or stuck float too. My 2 cents.

[K5owner]

I thought of fuel pump but PO replaced fuel pump a few years ago. Doesn't mean he used a good one or that it hasn't gone bad but that is a thought I have had as well. Right now I have the timing set to factory specs and I am only getting pinging under harder accel whereas when I had it set to 12 degrees(where the PO had it) I got pinging under light throttle and hard accel pretty bad. I have made progress in doing some of the things I listed but I want all the detonation gone. I am thinking of replacing the fuel pump to be sure, EGR valve, thermostat and MAP sensor just to be sure. I figure after 30 years some of those systems might be getting long in the tooth and need of replacement anyway. The carb either started the problems or revealed them as I think with the way it was running before being rebuilt hid the pinging. Because it does run much better now then it did.

[mattybilt79]

I would look more toward the distributor and timing related components. I don't remember what they call it on those years electronic spark controller some b.s. like that. If it has always pinged under load it makes sense, runs fine under no load and partial throttle but pings at speed, probably not advancing like it should. If that's a condition that exsisted pre and post carb rebuild I would start there. And like you said these are old components if the carb throttle shafts were worn out what shape is the distributor in?

[K5owner]

Originally Posted by mattybilt79

I would look more toward the distributor and timing related components. I don't remember what they call it on those years electronic spark controller some b.s. like that. If it has always pinged under load it makes sense, runs fine under no load and partial throttle but pings at speed, probably not advancing like it should. If that's a condition that exsisted pre and post carb rebuild I would start there. And like you said these are old components if the carb throttle shafts were worn out what shape is the distributor in?

I think I am getting too much advance under load and instead of backing off the advance like a vacuum advance would do as the vacuum drops off under heavy accel the electronic advance of the all electronic distributor isn't doing that. I think MAP sensor is to blame the more I look into it. But the other components could stand to be replaced just to be sure.

[K5owner]

Well the winner is Rich. Bad MAP Sensor or "Diff Press Sensor" as the under hood decal shows it. Replaced it and he pinging under load is gone. In fact it ran a little cooler up the Cajon pass today as well. Thanks guy I really appreciate it. I am still going to replace the EGR and Thermostat with the factory spec units just to have it all new and peace of mind.

[mattybilt79]

Good deal glad you got it fixed. Your out there in my old stomping grounds. I've got a couple of my old k5s that were sold to guys in the hi desert in Hesperia when I lived up there.

[K5owner]

Yeah I live in San Bernardino and work in Hesperia so it is a daily drive up and down the hill for work. Although it isn't my daily driver it is nice to have the extra room for work sometimes. I am relieved to have it running well again.

[rich weyand]

Excellent!