Resolved - 350 Running rough for no reason

[Lunatic]

This will be the second time in 4 years that this has happened. The first time I "knew" it was the timing chain (but wasn't) and installed a direct drive gear set. Re-timed and has been running fine since.

Just the other day, I was driving through a neighborhood, almost idling. I turned onto a street, gave a bit of gas. and the engine stuttered - And just like that, it is now running like poo.

I can testify that everything is operating smoothly (bearings, push rods, etc) by using an automotive stethoscope all over the top and bottom of the engine. I just cant reason "how" the timing jumped, nor can I find where it jumped to. All I can achieve at this point is a very rough idle and a QJ that backfires under heavy pedal with no load.

A bit of info for the engine: 350 with stock internals, SMI QJ, Edelbrock 3701 intake, Accel Correction 8141R dizzy cap, new AC Delco plain jane spark plugs, Proform noisey direct drive, vacuum lines less than a year old.


I can only come with 2 ideas, both ignition related. Either my timing mysteriously jumped somehow, or my stock dizzy went south.

So before I tear down my engine, I thought I'd come through and present the case to the brotherhood. Hopefully, someone can point out something I haven't thought of or considered looking at.


I hate asking for help, but here I am. Ask any questions and I'll respond quickly. Comments and suggestions are appreciated also.




Troy B
Ft Hood, TX



Response in 73-91 K5/Suburban/Truck (LINK) - A re-direct link has been added in this thread to "here".

imiceman44
Did you try to retime it?
I suspect the dizzy is not as tight as it should be and might have moved.
The gear drive jumping a tooth is most unlikely so I think you could skip that.


Lunatic
I've re-stabbed the dizzy quite a few times, yes. While I haven't tightened it in a final manner as of yet, the day this happened it was indeed tight.

And yup - Thats exactly why I installed the gear drive - Just saying "gear drive" rules it out. And if you ever were to question a gear drive, it would have to be the reason..... Because the water pump would be in pieces and the timing cover shredded !


I'll be checking my rotor and dizzy a bit closer tomorrow. Thanks for the response.


Response from Pirate 4x4 (LINK)

wingnuts514
I would start with the carb, next guess would be cap and rotor. good luck.

If it's HEI check the control module for corrosion, not a bad idea to just replace it.


Lunatic
Okay, thanks - I will take the carb to initial turns tomorrow and see what happens.

The QJ isnt even a month old from SMI and was running well. The cap is also just as new with no signs or odors. The rotor looks to be in good shape too, but I will remove it for better inspection tomorrow as well.


Isn't carb backfire a tell tale sign for timing being off ? I thought I read that somewhere a long time ago (Memory may be distorted at this point).


Thanks for the quick response, I'll provide feedback tomorrow.


Scooter402
I was thinking either the carb or the mechanical advance in the distributor stuck open...if its an old HEI, there may be a lot of crud in it.


Lunatic
While I'm under the cap tomorrow I'll look closely at the internals of the dizzy and give feedback.


Thanks for the response !


oldenginebuilder
Could be a vacuum leak from one of the intake gaskets. With the engine trying to idle, spray something flammable above each intake port and hope the idle improves.

If TH350, could be a bad vacuum regulator diaphragm, but then you should get some smoke out the exhaust.

Same deal but no smoke if it's a bad diaphragm for the vacuum advance.
Have an EGR valve on there?


Lunatic
The intake was installed not a month ago, for a second time within a weeks time. I didn't get a good seal on the rear right corner the first time. I am not suggesting the intake isn't a possibility, though - I'm just providing further information. I'll find something to shoot the ports with and give feedback.

"Just the other day" is actually 2 weeks ago - I haven't been able to get back to the issue until now. So, in a time frame perspective, this all happened approximately two weeks after I (1) replaced the dizzy cap with the corrective cap (to dress my cables), and (2) installed the new SMI carb and new intake. I had just finished tuning the carb (over a week of 30-40mph driving) when this occurred. The thing that blows is I was happy with it and ready to walk away. I guess the engine had other plans.

There is no vacuum on the dizzy.

There is an EGR - How would I check it ?

Let's talk about smoke out of the exhaust for a minute. It amuses me for some strange reason that you brought it up. I have very minimal smoke from my tips. BUT, I have a blow-by issue on both sides, right side being the worse of the two.

I'll shoot a picture of it and link it to my Bucket acct - I doubt I can post it here with under 20 posts. But, I have wondered if some rings gave up. I really need to run to Harbor Freight for a compression kit to check my cylinders but haven't yet. Maybe once I check all the previous suggestions I'll go pick one up.

The reason it amuses me though..... I've been looking at 383s all night.

Perhaps in anticipation of my current denial.




Thanks for the response !

 

[Fordum]

First of all, when you get it fixed, stay the heck out of that neighborhood!
Next, this is classic timing symptoms, but I once had a similar situation with a double barrel carb when one jet got clogged.
I would not tear the carb down looking for that at this point, but something to keep in mind.
Before you go too far, you need to find a timing light. That will tell you real quick if its a timing issue.
While looking under the hood, give a glance to the plug wires. Very unlikely while just driving down the road, but if the loom came apart and the wrong wires got next to each other, you could get some inductive crossfire.

My best guess right now, is timing. There are several things that could have happened.
Problems with the distributor. This is the most likely. Either it moved in the hole, or had some kind of internal breakdown. Rotor cracked, advance system broke. When you take the cap off, you should be able to turn the rotor a short distance and it should snap back.
If it stays, your advance is broken.

Next most likely, is the pin sheared on the dist. drive gear, letting it partially turn on the shaft.
Only seen that twice. And only once did the truck keep running. The other time, the gear just kept spinning and the truck stopped.

Lastly, and the hardest to check, is a partially sheared key on the timing gear.

All those will show up with a timing light. Then you just have to figure out which it is.

A really lean condition can cause the backfiring through the carb, which is why I mentioned the stopped up jet, but the ignition firing before the intake is completely closed is the most common.

Let us know what you find.

 

[Lunatic]

Thanks for the reply, Fordum.

I started this morning with the cap, and haven't gotten much further.

I found that the ignition coil ground wire is separated with only insulation holding it in place and another positive wire with stripped insulation. These wires were crushed by the coil cover.

I came inside to find local availability and grab my camera battery off the charger.

I do have (and use) a timing light and iirc was running around 4+ @ 700 rpm but don't quote me. Plug wires are in good shape.

I'm going to pick up a ignition coil and module here shortly, install, and see what happens.




Troy B
Ft Hood, TX

 

[imiceman44]

I think you found your culprit.

Thanks for the reply, Fordum.

I started this morning with the cap, and haven't gotten much further.

I found that the ignition coil ground wire is separated with only insulation holding it in place and another positive wire with stripped insulation. These wires were crushed by the coil cover.

I came inside to find local availability and grab my camera battery off the charger.

I do have (and use) a timing light and iirc was running around 4+ @ 700 rpm but don't quote me. Plug wires are in good shape.

I'm going to pick up a ignition coil and module here shortly, install, and see what happens.




Troy B
Ft Hood, TX

 

[Lunatic]

I think so too.

I'll post feedback once the new module and coil are installed.




Troy B
Ft Hood, TX

 

[Lunatic]

Play by play here..........

The module and coil are installed - It seems to be pretty smooth and responsive atm - Lost the carb backfire. I'm going to re-install my exhaust now so that I can hear better - Straight headers atm.

I'll report back shortly.


It was running smoothly until it got to operating temp. Backfire is back and a noticeable thump can be felt if I touch the AC comp. It doesn't have any particular rhythm to it but seems like a bearing on the way out perhaps.

Off to Harbor Freight


Troy B
Ft Hood, TX

 

[Lunatic]

^^^

 

[ryoken]

sounds like a miss.. did you do a plug reading?

 

[Lunatic]

So, the evening turned into a "take down Christmas lights and decor" at 2 houses.

So, in the morning, I'll be pulling all plugs for a comp test - I'll check their condition at that time.

Before I left, I did check the coil again to see if it fried - Still new.

Tomorrow presents a fresh start.


Thanks for the response, Ryoken





Troy B
Ft Hood, TX

 

[Amtek]

Have you checked fuel volume and pressure? Could be a clogged filter or pick up. It can still build up pressure and fill the fuel bowls but the volume is decreased and pessure drops under load.

I just rebuilt my engine and had this happen. The two years I drove it before I parked it, truck ran fine. So when i put it back together I figured the fuel system was fine too. Drop the engine back in one year later and suddenly one of filters is clogged, drove me crazy trying to find why I would randomly backfire through the carb.

Also, the EGR valve may be getting stuck open? I saw you ask about the EGR, but didnt say if you inspected or replaced it. If stuck open is like a big internal vacuum leak.

You also mentioned that you have a blow by issue? Is your PCV hooked up and working? you should hear rattling or a buzzing sound from it.

 

[Lunatic]

I believe this is resolved - A week of driving will tell all.

So, this morning I ran out and bought an inline spark plug tester before beginning anything. I came back and checked my odd side. It was so inconsistent that I decided to go another route.

I pulled my cap, broke all my plug wires loose of clamps and such, and swapped from the correction cap to my old regular.

Problem is now gone. I drove around the neighborhood and a few main streets checking for response and all the symptoms that had disappeared - I found none.

So, I timed it back to 4+ approx 750-800 rpm, turned my QJ back down to 2 turns each side, and shut down. I'll DD it this week and see what happens.

I appreciate everyone's input in helping lead to a solution. I'm glad I chose to post as a last effort before tearing down - It turned out much better than if I hadn't that's for sure.

I'm still going to run a compression test later - Just not today. I'm curious of bleed down and the condition of my cylinders.



Nothing to see here - All points look fine.

But if you look at the top, all but 2 points (1&4) are scarred in some way. I didn't realize the impact this would have. I do now !

The ground is hanging by insulation right below the burn in the positive wire.

Just eye candy of July's paint job.

Once again - Thank you, brotherhood !


I'll report back next weekend with feedback and hopefully put this behind me for good.



Troy B
Ft Hood, TX

 

[ryoken]

correct-a-caps suck, I don't care what anyone says.... Merc run's em on a couple of their packages, so does Volvo..... We had a HUGE recall on bad caps this year.... Sierra made em, thank god I had only installed em on 2 boats before I had an issue and caught it...

my buddy at the marina south of me had installed over 20 he had to go bavk and replace... same kinda sh*t, misfires under load, yada, yada..... some manufacturing error... bought Merc caps, all was good....

I blasted my parts distributor over that one...

 

[Lunatic]

Four days later and the engine is running smooth.

This weekend I'll fine tune the QJ and check my cylinder compression numbers.

Thanks, once again, to everyone that responded with suggestions.




Troy B
Ft Hood, TX

 

[p0ps45]

correction cap??

I gotta ask...whats a correction cap???

 

[diesel4me]

I belive its one of those custom dizzy caps, that re-route the plug wires so all the ones going to the right side bank point right,and the left side bank to the left,so you dont have them "crisss-crossed"....they improve the LOOKS,but not much else,judging by this thread,and Ryoken's experiences with them..

 

[p0ps45]

Sounds like a waste of $$'s and possibly a whole lot of problems! Thanks.

 

[Lunatic]

Yeah - Correction cap = 50 bucks wasted.

I'm strongly opposed to them now.



Troy B
Ft Hood, TX