84 4+3

Hello everyone. (If this is the wrong section please let me know by the way) So I am new here and well being on other forums I hate joining and then asking for help with a problem almost immediately thereafter but I mean that is what these are for I guess but I just feel kind of bad about it. So let me first begin with what caused this issue to arise. As a lot of you know the 84s are slow by today's standards. I get that. So before I did the little bit of work to it I did, I never really beat on the car so to speak. So in reality I don't know if my problem was occurring before I did this. Now the work I did was very mild. I swapped in a very mild cam and a renegade intake manifold as well as new lifters, rockers, timing chain, etc. My cam choice was made after careful research to make sure it would play well with the stock computer. It would. I ended up with a very very mild upgrade. It is comparable to the 89 l98 cam specs with a little more lift.

Next I made all of my adjustments then broke in the cam. After break in I set the base timing to 8 degrees and took it for a test drive. Had a check engine light. Okay. Code was for coolant sensor. I still had the old knock sensor style plug and it was questionable at best. Replaced it with a new style sensor and plug then it was good and went into closed loop with no more light. Awesome. Now I start driving and I can actually smell the car leaning out. Okay. Adjust fuel pressure. No good. New fuel pump fixed pressure issue but still a tad lean. So I swapped in larger injectors and dialed back the pressure. lean out was then fixed. Now the oxygen sensor isn't mapping great. Checked connections and they were good so I replaced the O2 sensor. Nice smooth voltage swings now.

Now all of this was occurring while I had a break up starting around 2500 rpms and clearing out around 3100-3300. The best way to describe it is that it feels like it is hitting a rev limiter. The car just stops pulling and it actually almost sounds like ignition is being cut to some cylinders. Alright. Check the plug wires. They got cooked during cam break in (manifolds turned cherry red. Like cherry red.) and were arcing. New wires helped a lot then the car really started bogging a day later. Culprit was the coil. New coil and it ran like a top again. New ignition module for safe measures since it was original. Still missing or breaking up starting at 2500. Checked vacuum 19.5 at idle. Soaked the intake in starter fluid with no change in rpms. No vacuum leaks... For the heck of it I swapped the plugs. I had heard of issues with the delco #2 plugs so i went with the regular coppers. (rs44t or something like that) Ran so much better but still does it. Balanced the throttle bodies and the idle became perfect but it still does it....

Now at this point the car behaves good. Linear throttle response and get this, it doesn't break up every time. I can take off from a light and max out first and it is good then shift second and then it hits 2500 and starts doing it. then short shift third and bury it again and it is fine. Other times I can row straight through third without missing a beat. I am literally losing it now.

To recap in a list what was done for reference:
-cap and rotor (old age mainly)
-coil
-plugs and wires
-O2 sensor
-balanced throttle bodies
-adjusted and checked throttle position sensor
-fuel pump and pressure adjusted
-Coolant temp sensor replaced
-Checked for vacuum leaks
-Firing order (just in case I messed up the 4 times I checked.)
-I may be forgetting more, it has been a month since I really tinkered with it or even drove it because this is getting annoying.

Now I know the throttle bodies are a little worn where the shaft goes through but I got them to hold balance pretty well. But if I am just mashing the skinny pedal a couple thousandths of difference between the two blades really shouldn't make a difference like this. I am at the point where I don't know what else to check. Maybe the pick up for the distributor maybe the ecm I really don't know what else to do. I am out of things I am willing to parts change as nothing else tests questionable in my opinion. The car runs perfect aside from the break up and a lot of times you can back off and then power through or just power through. It is really strange and no one can seem to figure out what the deal is. Any help is much appreciated and I will be getting my win aldl back soon so I can run some tests on that as well.

Thanks in advance.

Hot Rod Roy

Don't feel bad about asking for advice. That's why we're here. With your experience, you'll be offering help to others very soon!

Here's an easy test to rule out a vacuum leak in your vacuum accessory circuit. At the back of your intake manifold is the port for the vacuum accessories, and there's a bunch of them! This port goes to a vacuum check valve (or should). The vacuum in this line, after the check valve, should hold vacuum for quite a while!

The vacuum solenoid on your heater water valve should pull in when you start the engine (heater off), and should stay pulled in after the engine is shut off. Mine stays in for several hours! Does yours? If not, you've got a vacuum leak.

84 4+3

So that solenoid is actually stuck open, it failed actually. So as for the vacuum side, it holds vacuum after the check valve for a couple of minutes. If I sit in the driver's seat I can actually hear the vacuum actuators in the dash release after a minute or two. I ran through those tests while I was trying to figure out why my cruise control didn't work actually. I believe it's the line running through the firewall that is leaking but I still get the same vacuum reading at accssorys as I do at the intake. If you think that could be causing this I'll replace it sooner rather than over the winter like I had planned to.

Hot Rod Roy

For a test, you might try blocking off that vacuum line at the back of your intake manifold. Of course, non of your vacuum operated accessories will work, but it would be interesting to see if your performance improves.

On a separate note, the heater control valve on the '84 is a one-year wonder, and as such, they're hard to find, and expensive. I replaced the heater valve in my '84 with a Four-Seasons 74800. Here's a link to my old post:

https://www.corvetteforum.com/forums...ter-valve.html

One problem with the Renegade manifold is that there is no water flow from the back of the LH head into the intake manifold, like there is on the original CFI manifold. So I'm hoping the intake manifold gasket you used on the RH head has a full sized opening to the water port at the back of the RH head. Otherwise you won't have much hot water flow to your heater.

84 4+3

I'll try that later then. As for the gasket it is the oem style felpro. The manifold from the factory had the rear ports blocked with a piece of metal. I believe these had the opening at roughly half on the gasket. When I was breaking in the cam I let the efan take care of the temps for a while and it did a good job holding it to 230 in neutral on a 95 degree day but 230 scares me so I flipped on the heat and that brought it to 220 or so after the fan kicked off and then slowly climbed back up. I had a thermometer in the vent that was reading something to the tune of 190 from the heat so it seems to be working rather efficiently.

I will take a look at that valve replacement though. You know, nagging issues.

84 4+3

Weather is not cooperating. Have the line in the intake cap plugged that goes to the check valve but every time I rolled out of the garage it started raining heavily for the last hour and a half or so... I'm not about driving pedal to the medal on wet roads, things can get away very quickly like that. I'll probably end up testing tomorrow evening.

84 4+3

Update: took it out today. There was a lot of traffic in town. The little bit I got on it it seemed good. But here's the thing. We have very cool dry weather right now and it tends to be better on those days. I also readjusted my tps again (forgot to after throttle body balancing) it was close at about .6 volts but I set it back to .525. If I cruise at 2500 in 3rd say it still kind of burbles if you will so I don't think it's gone. Also I had the go pro running to try and video tape it happening but of course it'll never act up while that's on. When the card filled up that's when the burbling started of course. Always one nagging issue.

Buccaneer

84 4+3

That was the least of my problems with that manifold luckily... But that is a story for another day

84 4+3

Update 2: I was able to get it to do it. With the line attached and in attached. Actually on my drive I disconnected it while it was running and replugged the intake and when I got home I moved my hvac to defrost and it worked. So it held vacuum on the accessory side for my 10 minute drive. I'm going to guess there isn't a leak there after all. No when I got it to do it it wasn't so much wide open more like part throttle. I'm working on a video clip of it now, not that it'll help much.

Hot Rod Roy

Buccaneer, I'm not sure I understand what you're saying, with the popcorn icon. Can you please elaborate? I value your comments! I enjoyed reading about your '82 project.

84 4+3

So no, it hasn't gone away I have just been busy. Here is the video I was talking about. You really can not tell it happened from this though. It's really annoying me... You can hear the exhaust note change right before I let off at the end. That is when it sounds right and feels right if that makes sense. You can hear it like, jump up a little in rpms even and you definitely feel it.

corvettenorway

bad timing, maybe a bad balancer ?

ex-x-fire

Pop the cap off of the distributor, grab the rotor, & see if there's any slop there.

84 4+3

I tried 6 degrees base and up to 9 degrees with no improvement.
Axial or sideways I assume? There is some end play as I recall but I didn't notice any side to side play last time I was in there. I will check more thoroughly and report back as I think the spec for end play is rather small couple hundredths or something like that? As well as horizontal play.

Just some speculation on my part. I had read that the 624 heads are known for cracking under high heat. Is it not possible I cracked the head during cam break in? I mean the manifolds as I was told looked cherry red and looked like that all the way down to the Y pipe. Heck the stainless on the rear changed to a slight gold color from all the heat and the exhaust tips melted the paint on the very inside of the cutout. Maybe I broke something then. (Looking for an excuse to buy new heads. )

84 4+3

Update: seems worse today. No appreciable play in distributor. Will assess further.

Hot Rod Roy

Yike! Why would you let that happen? "As I was told"? Somebody else did that? This was someone you apparently trusted? Not a good sign! What else did they work on?

Have you done a compression test?

84 4+3

Well, I was told once you start the cam break in you aren't supposed to stop. Being I was in the driver's seat I couldn't see what was going on under the hood. I had three people watching, I figured it was normal. They always glowed a little after I drove so I figured 95 degree ambient temp plus 230 coolant temp plus 25 minutes around 2500 rpm equals they'd be a little red sitting in my driveway. I did do a compression test before the work and got 148 across all 8 give or take. My math says that is about right for a small block with 100k on it. I'll run a compression test again later to see where it is at... the thought only crossed my mind the other day at school.

Hot Rod Roy

You're supposed to have a timing light on a new start-up, so you can correct the timing promptly! Some lessons are learned the hard way! Good luck!

84 4+3

Hey, I wasn't too far off. It was about 0 or so. I was happy the thing started right up and ran. But yea some said throw the light on some didn't. It advanced to 30 or so according to the light so I assumed it was close. I think the problem ultimately was it never went to closed loop. (The knock sensor style connector on the temp sensor on the manifold. I said to myself it worked great for the last 10 years why change it.) I've never torn into a motor before. Being I just winged it by myself I was pretty happy. I know more now though. I was planning for heads over the winter so not a total loss. You live and learn.