GM 7-pin HEI help?

[John Milmont]

Guys,

I have been having a lot of trouble with my old 3rd gen f-body race car and have been trying to track down various issues for quite some time now.

I am on a new short block and, of course, using all of the old accessories. Using an IR heat gun, I have noticed that cylinder # 7 is very cold compared to the other cylinders. At idle, its only getting up to about 200 degrees where the other cylinders are in the upper 500's, measured at the exhaust header.

After testing everything: injectors, plugs, wires and compression, the only thing that seems to make sense is the distributor.

I took off the cap and it definitely needs to be replaced as some of the posts seem to be blasted pretty well. But the strange thing is that the post for cylinder #7 doesn't seem to be as bad as the others.

This lead me into thinking that, for some reason, #7 isn't getting the voltage the other cylinders are getting, or not firing as often.

What could be causing this? Could it be that the magnetic pickup in the distributor itself is the problem? I guess I'm going to replace the cap and rotor first, but I'm trying to figure out when its time to replace the entire distributor...

 

[John Milmont]

Here is one of the posts in the dizzy..

 

[Randy V]

Pull a compression test. The lower the compression, the lower the resistance to firing the plug. That might account for part of it.
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Try swapping the plug wires from cyl 5 and 7
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Injector possibly leaking on cyl 7 which would cause that cylinder to run cooler. Swap injectors and see if the problem follows,
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I have recently seen a reluctor wheel that was cracked which causes a weird effect on the hall effect sensor. This is particularly true at higher rpm rather than at idle speeds though.

 

[John Milmont]

Compression is good. 195psi on the cold cylinder.

I have tried swapping plugs, wires and injectors and injector connectors. No differences.

Spark is visible when I pulled the plug and grounded it.

It seems like the cylinder misses - when I pulled the plug, it was wet.

 

[John Milmont]

This is the post for cylinder #7

 

[Wally]

Hi John In the picture it looks like to rotor is running real close to the cap ??? is it hitting on the post just before cyl. 7 is there any contact markings the cap or rotor ???? Wally

 

[John Milmont]

We did notice that it was running really high, too. The first order of business will be to replace the cap and rotor and make sure everything is set correctly.

 

[Wally]

Pull the distributor and set it 90 degrees off and reset the wiring to the new possitions and see if the former #7 lead problem has now has moved to the cylinder that is now attached, That will tell you if it's in the distributor or cylinder 7. Also spray ( very carefully ) some starting fluid around the intake manifold and see if the RPM's change, We have had leaky intakes create these problems. Wally

 

[Jim Cowden]

John

195 on the one cylinder.
What where all the others?.

Valve clearance (not enough)if adjustable will do this.
It will give comp at crank speed but loose comp at idle speed.

Jim

 

[John Milmont]

All the others were in acceptable ranges. The lowest is cylinder number 1 at 180psi. Did a leak down test today and all cylinders were 6-8% leak down rate. I hope it gets better as the engine breaks in.

At this point, I'm pretty sure its the reluctor in the distributor. As Randy said, the GM small cap HEI distributors have a ceramic magnet which is known to crack. I found this is a known issue and can cause this cold cylinder problem. I think this is the issue. There does seem to be some strange fluctuations with the timing light.

You cannot check this problem by rotating the rotor and setting another post to cylinder #1. This is because the damage is on the reluctor pinwheel and will always cause a poor signal for whatever tip is cracked, so it will always be cylinder 7 in this case.

 

[John Milmont]

maybe an issue?

 

[Randy V]

Doesn't look good...

$75 and it is all new, comes with cap & rotor - done..
More Information for WORLD POWER SYSTEMS DST1830

 

[John Milmont]

already got a new one. just have to install it.

 

[John Milmont]

Just replaced the distributor and have observed no difference. Cylinders 1 and 7 seem to average 300 degrees colder than 3 and 5. Its hard to take a measure off the even side of the engine, but the even cylinders seem to be very close to each other.

When driving around 2200rpm or so, the car surges and bucks even though my AFR gauge is reading a constant value. It must be missing still, but I have no idea whats wrong.

 

[Randy V]

It almost has to be the injectors then.. Swap one with another cylinder and see if the problem follows..

 

[John Milmont]

I have swapped the injectors multiple times, and changed the connectors on them from one cylinder to another. The engine is batch fire, so it fires either the left or right bank so that's how I can do this.

Nothing has given me any results. 1 and 7 are still cold.

 

[Randy V]

An equal amount of fuel and air augmented by an equal amount of compression and spark timed appropriately, should deliver the same amount of heat energy. I think I would have to have this one on a dyno and do a power balance test. What is really odd is that the two cylinders are on the same bank but at opposite ends of the engine.

 

[Jim Cowden]

Sounds like its lean.
02 senser on an engine with a cam may give a false reading at 2200rpm due to overlap.

I would add fuel across the board at that rpm and see if the bucking goes away.
If not go the other way and just feel what it likes John.
Use the force.

Easy to get fooled with your meter if its to lean it will lean missfire and not burn the fuel, the
meter will tell you its 10:1 because the hydrocarbons are unburnt and are in the tail pipe.
Try rich and come down its safer.

I assume you have no air leaks.
V8s sometimes suck air from in the valley.
I spray aerostart or carby clean into the rocker cover and see if it makes a difference.
A valley leak will pull the spray in and raise the idle if its leaking.

Some of this will give you more info to work with.


Worth a try

 

[John Milmont]

the cam is a comp cams 280 xfi, 230/236 i think we are are around .57 lift with our 1.6 rockers. this cam is supposed to be quite driveable on a 383, so i dont think these issues are due to low rpm problems big cams bring.


we have checked for airleaks like crazy. i guess it wouldnt hurt to check for more. This is a completely new block, and even though we are using the same heads and intake, im pretty sure its not an air leak because the previous engine ran poorly like this as well. we always thought it was a tuning issue. i never measured the exhaust temps then, but this engine pretty much behaves identically even though we are now using a completely stand alone computer sys.

i'm thinking that this may be a wiring problem since the wiring harnesses is not only old, but has been hacked up over the years. I'm wondering if it may be an interference or something.