Help me diagnose an intermittent stumble

It was a beautiful weekend and I took the 308 on a 100+ mile Mom's day visit. On the way there, I noticed an occasional little burble in the exhaust note. On the way back, it turned into an intermittent full-on loss of power and 1-4 slow down light on. The strange thing is that it would run fine for 5-10 minutes, stumble and backfire for a few seconds, then recover and run fine. My wife and I were both prepped for a ride home on a wrecker but I was able to nurse it home under it's own power.

We got home late and I have not had a chance to dive in yet to do any troubleshooting, but I wanted to consult with fellow F-Chatters. Where to start looking? It seems like an ignition problem on the 1-4 bank, but I can't be sure. Since it's an intermittent problem, I'm sure it will be a bugger to track down. Coils? Wires? Ignition pickup? XR-700 unit? When one of these fails, would it be intermittent or just stop working completely?

The car: 1978 308 GTS USA
The carbs: Recently rebuilt DCNFs
The pump: New pierburg
The plugs: NGK BP6ES
The wires: OEM (old)
The ignition: Allison (crane) XR-700
The pickups: Unknown in distributors

 

In all my years playing with cars I never had a bad set of plug wires but the orig set in my 308 caused misfires(sudden drop in revs), crossfire (burbles on decel, backfire, and flames out the exhaust(would have liked to keep this symptom, everyone said it looked cool) & fouled plugs. A new aftermarket set did the trick for me. I'd change the plugs too.
Bill

 

Sounds like its dropping a wwhole bank - the slow down light is not good, it means the Cat is getting too hot - usually caused by unburned fuel reaching the cat.

Not 100% clear on the setup you have but you need to start looking for things that affect a whole bank. In the standard setup that would be somewhere in the chain cap/rotor arm/coil lead/coil/crank sensor.

Are the crank sensors relevant in the setup you have?

 

It's hard to tell if I temporarily lost the 1-4 bank, or if it was just misfire on a few cylinders. I don't know what it feels or sounds like when the entire bank goes out. Seeing that slow down light sure scared me though.

The PO removed all the points, installed pickups in the distributors and two Allison (crane) XR-700 ignition modules. It does not use a crank sensor. I don't know much about this setup but I'm sure I'll be learning more real soon. I guess I'll start with pulling the plugs and the distributor caps. Maybe it's bad wires, burnt rotor, etc, etc...Hopefully, it's something obvious.

I almost wish that something would have failed completely. Chasing down stuff that MOSTLY works but SOMETIMES doesn't is a PITA.

 

Make sure your advance mechanism in that distributor isn't sticky, as long as you have it apart. Retarded ignition can cause problem like this. At least you know which bank to investigate.

Doug

 

I just had this exact situation happen in my 1978 308 GTS which has two XR-700 ignition boxes! First, the symptom was intermittent popping and backfiring, then my tachometer stopped functioning and I lost my forward bank of cylinders.

This happend on the Merrit 3 week ago, and after waiting for the flatbed and returning to Westchester, the car started right up and ran fine again for a minute or two when I drove it off the flatbed.

The replacement XR-700 box was $175.00

Good luck

 

Wow, what are the odds of someone with the same car, the same ignition setup and the same problem? Which XR-700 was the replacement? Crane's website shows a number of sub-model numbers for the XR-700. How much other troubleshooting did you do before coming to the conclusion that it was a bad ignition box?

 

I laughed myself when I saw your post! At first I thought it was an ignition coil as my wires, plugs, caps, and rotors are all new, and the car was running pretty well up until the ignition box failure. The number for the replacement Crane box is CRN7000231004.

An interesting side effect was my front bank was all out of whack due to the failing box, and the car is running amazing now! Faster than ever since I've owned it.

Did your Tach stop working as well?

 

The XR-700 was originally called the Allison, I don't know what Crane did when they took over Allison but my Alisson XR-700 lasted 15 years with no problem I had them on my Fiat/Lancia
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Honestly, I was so busy freaking out that I didn't notice if the tach stopped working or not. So you only replaced one of the boxes, yes?

 

Yep, that's pretty much what mine looks like. Mine are both rep-Crane Allison units. Do you have two of these, one for each bank?

 

That's correct, I replaced just one of the boxes and my car is running better than ever.

My mechanic also disconnected the charcoal canister on my car as it was clogged, and it seems to make a difference as well.

 

Cool. My Allison XR-700 boxes are really old and I wonder how they differ from the current Crane XR-700. If I do end up needing to replace one, perhaps I'd be better off replacing both? Brian, what is your static timing set to under this setup?

I did spend a few moments last night to pull the 1-4 distributor cap. The cap and rotor appeared ok -- at least nothing obvious like burnt up components. There are no loose wiring connections either. Initial signs seem to point at the XR-700 box.

 

I actually have that information in my maintenance files at home, and will try to pull it for you this evening. Brian Vignale, of Independent Motor works, Short Hills, NJ is the person who did an absolutely wonderful job on my cars conversion. You can't see anything, and all the original components are still there including the green wires that go into the caps.
I'll get his number for you as well if you like.

I replaced my cap and rotors with Superperformance products, and saved all the originals even though they were fine. Check the little buttons inside the cap for the coil wire as they can wear down and leave the rest looking ok.

Lots of fun right!

 

The caps are rebuildable.......

 

How? I suppose it's possible if you need to replace the carbon button or something, but I think you're SOL if the cap is cracked.

 

My 78 308GTS had the same set up...it was even serviced and installed by Ray at British Italian just before he moved to Idaho. The car is still around...someone said it is in the Snohomish area, so be on the look out for your twin! ( I almost thought that was my old car until I saw the engine compartment sorry...) Mine was a total "garage queen" owned by a friend of mine for many years, who rarely drove it and always kept it covered. It was a sweet car.

 

There seem to be quite a few '78 308s in the area. Strange. My car is definitely not a garage queen. Under her former owners, she was driven hard and put away wet. This is apparently the car Ray had in storage for many many years, after the owners walked away.

On a relevant note, I decided to pick up a new Crane XR-700. According to the Crane guy I talked to, the current XR-700 has exactly the same performance characteristics as the old Allison XR-700. Only $110 at Summit. Hopefully, this will fix my problem.