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TBI intermittent starting?

Mastiff

Mastiff

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So my 83 K5 has been upfitted with GM TBI and it has been working great for years. At this point the truck mostly sits, with short trips now and then. I recently noticed it was taking more cranking to start than usual, and then last night we wanted to take it somewhere and it wouldn't start at all. I could hear the pump priming before start and it cranks like a champ, but absolutely no sign of spark (from the driver's seat). On a lark, I tried again this morning and it pretty much fired right up. The weather was mild both times.

Any ideas what the cause of this could be? I was thinking maybe the module in the distributor, since that's original from 1983, but I would expect that to be all or nothing, not intermittent. Sticky relay?
 
Pump could be spinning but not pushing. Have a pressure gage?
 
Bent77 said:
Pump could be spinning but not pushing. Have a pressure gage?
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I think somewhere. Kind of a PITA since you have to break the connections and T it in somehow. I'm not sure how it could not start one time and then run fine the next if the pump was shot.
 
I made adapters to take the fuel filters place, a flow through guage so truck will still run.
 
My first bet would be the module, or the pick up coil wires,
2nd is the injector wiring.
3rd if the iac full closed won't start.
When not starting If you crack the throttle and it starts the iac is/was stuck.
 
Wes Harden said:
My first bet would be the module, or the pick up coil wires,
2nd is the injector wiring.
3rd if the iac full closed won't start.
When not starting If you crack the throttle and it starts the iac is/was stuck.
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Do you think a bad module could cause it to start hard but then run fine once started? I'm more than happy to swap the module since it's so old anyway, if there's a possibility it could cause what I'm seeing.
 
Mastiff said:
I was thinking maybe the module in the distributor, since that's original from 1983, but I would expect that to be all or nothing, not intermittent. Sticky relay?
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Are you using a mechanical distributor?

Martin
 
Hard to say, electronic modules do funny things. I would expect it to fail on longer trips, but sort drives yeah it could work.

At the very least I would inspect the internals of the dist esp the wiring and connections. Same for the injectors.
 
I'm a bit confused as to how and why you'd end up with mechanical timing on a system built to do so electrically.

Reason I mention it is that if ignition components are suspect, anymore it's cheaper to just get a whole new distributor and call it a day. If going to the TBI distributor at this point makes sense, that's what I'd do when replacing. Pickup coil and ignition module are both suspect in old age, replacing just the module and finding out it's the pickup coil is a fools errand. Ask how I know lol.
 
I think I misspoke. It's been a long time since I did this install. I did replace the distributor with a TBI one, but kept my coil in cap. I think there's no such thing as a pickup coil in the TBI distributor, right? But still a module of some kind?

Anyway, it's frustrating because now it is starting even though last night it would not start for anything. While poking around, I noticed these rubber caps were cracked, one of them badly enough to definitely allow air in. Anyone know their purpose? They've always been plugged since I did this upgrade.

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I wish GM had made the original Hei the tbi computer controlled distributor. The newer small dists always failed much sooner the larger older brother.
 
those are for emission control vacuum controls. replace the caps with nice quality vinyl ones.
 
All HEI has a pickup coil. And you have to remove the distributor to get at it. Which is yet another reason to just replace the whole thing if you suspect components and don't want to test each one (which is possible, but intermittents are a real bear to diagnose) . For the price, you know you'd at least end up with a spare that generally works lol.

My 1988 distributor (external coil from an f-body setup, all innards) lasted about 20 years before failing. Since it's damp here and my truck rarely gets driven, I suspect that accelerated all the corrosion I found on the pickup, and it probably would have gone longer save for that.

Personally I prefer the small caps, to me they feel easier to deal with. Even in the truck engine bay, the large caps are pretty close to the firewall IIRC. External coil is nice for being able to easily replace. For the number of times you have to do it? Probably not worth a swap for future ease of repair. But since I had those parts, I went that way.

FWIW, on any setup I planned to run for awhile, I pulled extraneous fittings and tapped the holes for NPT. Never going to degrade or blow off. Plus, could always get a thread in fitting if you needed the port in the future.
 
I will tell you that I put a new distributor in my '95 truck and the module began to fail in under 10 miles. Barely got back to the house. I installed the Delco one from the old dizzy, and it went 2 more years.
The Specter module also failed in a buddy's truck.
 
Well, so far it keeps starting fine since I replaced those rubber caps. I'm not totally convinced that was the whole problem, but fingers crossed.

I'll go for a Delco module if I need to replace. I've read they can overheat too, if not installed properly.
 
The aftermarket modules have been problematic for some time. With the counterfeits anymore, need to be careful buying "Delco", and try to get it from a reputable source. Counterfeits are making it into major retailers, but I tend to think major retailers are a bit closer to the source, so less likely to sneak in a knockoff. eBay or Amazon not so much.

Id want to look up the part numbers before just grabbing random modules, as there are differences, but I'd rather have a used Delco module than a new aftermarket honestly. But for convenience, the aftermarket just (theoretically) eliminates the module and pickup at the same time. The modules used to be one of the items I grabbed when I saw them in the wrecking yards. I have no problem using old stuff like that, no doubt in my mind they were built better back then.
 
Mastiff said:
Well, so far it keeps starting fine since I replaced those rubber caps. I'm not totally convinced that was the whole problem, but fingers crossed.

I'll go for a Delco module if I need to replace. I've read they can overheat too, if not installed properly.
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Those caps are a vacuum leak so would definitely cause a problem. But did you do some oddball conversion using and in-cap coil? I didn’t realize you could do that, or I might have done it when I converted mine.
 
bp71k5 said:
Those caps are a vacuum leak so would definitely cause a problem. But did you do some oddball conversion using and in-cap coil? I didn’t realize you could do that, or I might have done it when I converted mine.
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Yeah, I had some fly by night guy help build up my conversion many years ago. One of the things he did was keep the coil in cap, though the distributor body is from TBI. I guess it's nice not to have the coil mounted someplace, but the weird wiring makes it hard to get help or troubleshoot compared to factory wiring. I should have gone with Howell.
 
Mastiff said:
Yeah, I had some fly by night guy help build up my conversion many years ago. One of the things he did was keep the coil in cap, though the distributor body is from TBI. I guess it's nice not to have the coil mounted someplace, but the weird wiring makes it hard to get help or troubleshoot compared to factory wiring. I should have gone with Howell.
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Ah. I see. I just made a bracket to hold the external coil with stock TBI distributor.
 
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