Posted this in the general engine thread 2 years ago, can be seen here: https://www.thirdgen.org/forums/tech...connected.html

2 years later, a new engine, bunch of things replaced and it's still there. 87 gta. Originally 350 tpi but swapped over to a stealth ram vortec 350 a few weeks ago.

With the EST wire unplugged, it's still there but not as profound, plug in the EST wire and it amplifies it. I can't for the life of me figure this out, especially on a new engine. Replaced every sensor except for the coolant temp switch and knock sensor but they tested good with meters and respond appropriately when data-logging. Replaced ESC module, coil, complete new distributor, ECM with PROM, new injectors, new TPS and IAC. Cold start it doesn't seem as bad but after it's warmed up it gets pretty bad.

Here's a video of what it's doing. This is with EST connected.

 

What is your timing at when idling? I don't just mean base timing, with advance too. Did you reuse that balancer? Maybe it spun and the timing is set a little off?

 

Going by my log with DataMaster, at idle timing with advance is at 20*. This is a completely different engine. Nothing from the old engine was used.

 

Not that I think it's off but I'd index the balancer or use a dial back timing light to verify that the computer is right. That would be to put my mind at ease, I often don't trust what ecm's "think".

Did the miss start before or after that exhaust got put on? I know it sounds dumb but my old 305 in a 86 gmc van idled funny till I put a balance tube between banks. Maybe that exhaust is causing a strange scavenge (or lack thereof) effect. Just tossing out some ideas.

 

I'm open to anything now so I do appreciate it. I put a timing light on it for base timing but not with advance timing, will have to look into that.

It seems this happened 2 years ago on my old engine. I decided to replace the distributor and then noticed the miss. It was an autozone one. Put my old one back in and that seemed to have taken care of it at the time. I've put the stock manifolds back on with stock Y to a cherry bomb and it was the same way.

 

Had suspicions of the fuel pump so replaced that with the Racetronix kit and wiring harness. No change at all. I'm running with the EST wire disconnected cause that's the only way it'll run decent. Plugging it in the idle will tend to surge and even flat out cut off on me a few times. Started right back up though. Really could use any insight with this.

 

You said you had a new distributor installed then put your old one back in. Have you tried replacing the ICM with a new GM unit. I no longer trust aftermarket ICM.

Also, have you checked voltage going to the ecm. I saw a post from another member with an idle problem and he finally discovered it was caused by low voltage, about 11 volts, coming from the alternator.

 

Thanks for the help Fred. The new distributor I was able to return so I put my old one back in. I have 2 used complete distributors and they both produce the same result.

Good point there with the voltage. I actually checked that against my factory service manual. Dropped the ECM and probed the back harness with KOEO and KOER and all voltages were within spec.

This is making me think it's ignition/spark related just not sure what. The fact that it does it even with the EST disconnected is a real head scratcher. It's like plugging the EST wire in for computer timing and it makes it that much worse.

 

I would connect my ignition scope and look at KV patterns to see if the misfire is displayed in the KV signature. Try holding a timing light to the balancer and see if timing is changing as the misfire occurs. One thing aside from ignition that can cause that kind of random misfire is lean AFR. Make sure that the engine isn't running lean.

 

Thanks for the input. I don't have a ignition scope but have gone around every cylinder with a timing light and noticed no change. I have datalogged quite a few times with DataMaster and per that it isn't running lean. I have a log if you'd like to take a look at it. I have fuel pressure at roughly 35psi at idle which is where my problem is.

 

One thing I have seen on a few Vortec V8s is dist. gear wear. If you've already checked for this, that's all good. The Vortec dist. has no mech advance and the rotor should have very little lash. If you can rock the rotor back and forth very much, suspect a worn gear. In stock form, they would set a fault code for cam/crank sync if the problem got bad enough. The worn gear allows too much timing fluctuation and can lead to this kind of intermittent misfire.

Another likely suspect would be the O2 sensor. If it's reading off a little, the system could be going too lean when the Integrator goes low.

Just some food for thought.

 

I'm actually using my stock oem tpi distributor. Kept the swap simple as I could.

I also suspected the O2 sensor and even replaced it (running a 3 wire one) but that made no difference. Especially since it also does it cold in open loop. I've datalogged with DataMaster and the BLMs at idle are roughly in the low 130 range.
this is definitely mind boggling.

 

If I knew that the fuel trims are in range and that there isn't any ignition fault, I would look at the valve train. Specifically valve guide wear, or out of round sets and faces. This kind of wear will cause variations in cylinder sealing that are most noticeable at idle. To check is as easy as pulling a valve cover, a couple of rockers and springs. I know it's odd that the same problem followed the new engine into the car. Makes me think more of an ignition or fueling issue, but the kind of valvetrain wear that I'm talking about is a big problem on GM gen SBC engines. The Vortec shares the same basic valvetrain design as the gen 1 and has the same flaws. Fortunately, a thorough valve job on the SBC cast heads is pretty cheap.

The test for valvetrain faults, without pulling a valve cover, is running compression test. I have a transducer that works with my labscope to make a nice picture of the compression cycle. It helps me to see a poorly sealing valve. Still, I tend to pull a valve cover if I have any question. Another way to locate one cylinder with an intermittent issue is cylinder kill. One cylinder at a time, pull the injector connector, pull and ground the plug wire. Then start the motor and see if the misfire has become a single steady miss. If it has, that's the problem cylinder.

Did you mention having the injectors checked? A dribbling nozzle or two, or faulty spray patterns, could cause this same effect. An injector service center will test them on an injector bench that tests the injector for every type of fault, then clean and flow test them. And it's alot less money than new injectors.

 

Just wanted to give an update on a discovery I found. I stumbled across and old school trick. The dollar bill test as it's commonly known. With engine running at idle, you put a dollar bill or piece of paper or even a rag up to the exhaust pipe. If it wants to suck it in, you have a exhaust valve problem. Whether burnt or staying open, etc. My dollar bill was going nuts. When I recently built this engine I took a stab at adjusting valves for the very first time. Used a video I found on YouTube for the adjustments. Also the heads were completely rebuilt.

I set the valves on the engine stand with each piston at TDC and left it at that. Never revisited them. I have a pretty strong feeling they may be out of proper adjustment now. Especially since there is a well known method that's used to adjust valves called EOIC (exhaust opening, intake closing) that I had no clue about.

 

What did you set the valves at? Is this a hydraulic or solid cam?

I've been using the EOIC method for the past year. Done several engines that way now. I understand how it is much more definite that the TDC method I used for years before. With a large cam, lots of overlap, even at TDC the lobes may not be on base circle. Comp Cams recommends the EOIC method for all their cams.

 

I initially set them at 1/2 a turn and left it at that. This was when it was on the engine stand. It's a hydraulic cam. Nothing fancy, 93 lt1 f-body.

Did the valve adjustments with car running this time warmed up, made absolutely no change.

 

Put in a brand new distributor today as I was using the old one from when I first got the car a few years back, made no change.

 

I quit adjusting valves with the engine running a long time ago. Aside from the mess, I don't see how anyone can be sure of accurate adjustment when trusting sound as a basis for zero lash. I use the exhaust opening, intake closing method. In short, you rotate the engine by hand until the exhaust valve just starts to open and set the intake valve at that point, then you rotate the engine until the intake valve is just about to close and set the exhaust valve at that point. 1/2 turn is good for most performance engines.

This method is recommended by Comp Cams for all of their cams, hydraulic or solid. You can find it on the instructions page of their website.

Your roving misfire is strange for sure. It doesn't seem to be ignition or valvetrain related. I wonder if just a little boost in idle fuel would help it. With a free flowing exhaust, you AFR is going to tend toward the lean side. Maybe as it toggles in closed loop, it gets just lean enough on the low side of the curve to get into lean misfire. Having the open exhaust is going to make this more noticeable than if it had a more restrictive, stockish exhaust.

 

I guess adjusting was just a easier method of the two, mess was controlled by a cardboard splash shield I made up, nothing fancy.

Recently I had a new Accufab regulator on there and turned up the pressure, no change at all.

Threw in a reman set of Bosch 3 24# injectors, aside from them running on the 30# tune, that also made no change. Swapped those in cause I questioned the 30# since getting them used with my HSR.

I have 2 videos, one from 2013 and the more recent one I took of the exhaust listening for the miss. Aside from having different exhaust setups, the miss is identical between the 2. I just don't get it. Could I have 2 totally different engines that have the same mechanical problem or is it something engine control wise that's causing it. That's the big question.