Ask Away! with Jeff Smith: Solving a Shaking, Hiccupping 383 Small Block Chevy in a 1971 Camaro

I have a problem that hopefully you can help. We just finished dropping my brand new 383 small block Chevy into my ’71 Camaro. I had a professional shop build the short block but my son and I assembled the top end of the engine and then dropped it into the car. It runs really nice and we spent about 10 minutes running it at fast idle to break in the engine. It has a hydraulic roller cam but we still felt it necessary to do the break-in and from your previous recommendations we used the Driven Break-In 30w oil.

The only problem is the engine has a weird little hiccup or cough at various engine speeds. Almost like a mini-backfire. And it seems like the engine is shaking—almost like it’s not balanced properly or something. I really don’t want to take this engine back out of the car. Any recommendations? 

H.M.

As a point to the readers of this column, I took the time to respond to this email with a follow-up as I had a couple of questions related to how and exactly what this “cough” sounded like, since I have to diagnose this without the luxury of listening to the engine. After a couple of short emails, I suggested a solution that we will get to in a moment.

First, I’d like to emphasize here that car and engine problem solving is as much as art as it is a science. Too often, we jump to conclusions without sufficient evidence and end up on a wild goose chase that does little to solve the problem. This can often involve expensive purchases of parts or tools that end up being unnecessary. Multiple hard-earned lessons have painfully taught me to always attempt the simple solutions first.

This car builder assumed the worst—that his new engine must be exhibiting some dire problem that will be expensive to repair.

The point is to rely less on hunches or guesses, and more on data or clues supplied by the engine.

Start Simple: Ignition Wires

My suggestion was to first check the simple things first. In this case, because it was a small block Chevy, a common problem that surfaces is mistakenly swapping the spark plug wires on cylinders 5 and 7. (I can’t tell you how many times I’ve personally made that mistake.)

The firing order on small and big block Chevy engines is 1-8-4-3-6-5-7-2. This means the plug wires on the distributor cap for 5 and 7 are adjacent, as are the cylinders and spark plugs because the Chevy cylinder order numbers the odd cylinders on the driver side with 1-3-5-7.

This makes it easy to swap the connections on either end of the wires.

The owner stated that the engine seemed to idle but had a mysterious “cough” at steady state. The mistake can be made in two ways. Either that 5 and 7 are mixed at the cap or that they are swapped at the spark plug ends. Either way, this sets up a situation where the spark plug in cylinder 7 fires before the plug in cylinder 5

In this case, the spark plug on cylinder 7 fires while the intake valve is either closed or just about to close based on cam timing. In this case, it wasn’t enough to create a backfire, but much of that is dependent upon cam and ignition timing. In this case, it caused a slight misfire and certainly the engine wasn’t running nearly as smoothly as it should. Likely the shake that the owner experienced was an imbalance caused by the very early firing of the number 5 spark plug wire plugged into the number 7 cylinder. For cylinder number 5, its spark plug fired 90 degrees too late and perhaps might have also contributed to the engine shake.

Switching any other cylinders probably would have caused either an outright backfire through the exhaust or out through the intake, but in this case, that did not occur. Large displacement engines like a 383 stroker, especially those with an aftermarket cam, can often mask a misfiring cylinder to make it appear that the engine is running okay.

Using a Thermal Gun to Check a Misfiring Cylinder

Another quick way to diagnose this problem (if the car has headers), is to use a thermal gun on each exhaust header pipe. The header pipe temperature should be within 20 to 30 degrees of each other. If you discover two cylinders adjacent to each other that are 200 degrees or so cooler, then it’s a good indication the two cylinders are misfiring.

The Result?

The easy fix in this case is to trace the spark plug wires to make sure they are routed correctly. And that’s when the owner discovered his error. We know this because he contacted us after repairing the mistake and reported that now the engine runs very nicely and has plenty of power.

We also mentioned to him that it is imperative that he immediately put the car on the road and load the engine with multiple quarter- to half-throttle acceleration bursts for three to five seconds for a few minutes. The load will immediately put cylinder pressure on the rings to help seat the rings to the cylinder wall. Just running the engine at fast idle with no load will not seat the rings properly. More importantly this insufficient load could cause the cylinder wall to glaze and not allow the rings to fully seat. Load is important as early as possible in a new engine’s life to ensure that the rings seat properly.

Start with the Basics

But back to the original problem, our emphasis for anybody working on cars in general is to always address the simple solutions first. It’s wasted effort to rebuild the carburetor when the real problem was just that the fuel tank was dry!