Intermittent power loss (need help troubleshooting please)

[trevarthan]

Hello,

I've got a 1971 c10 lwb with the 4.1L straight six.

Recently it has started losing power after driving for awhile (random intervals - as much as an hour, as little as a few minutes). Sometimes this is a complete stall, but sometimes it's just a lack of power. Sometimes it stutters when it loses power. Sometimes the engine keeps running when it loses power, but it just won't go.

It seems to be more likely to happen when the engine is warm, but I'm not 100% sure it matters. It'll stall at idle sometimes too if you restart it immediately after a stall and let it idle. Restarts are usually difficult after a stall.

We thought it might be a clogged carb fuel filter, so we removed the b-nut and changed the filter:





Unfortunately, it's still doing the same thing. So that's one thing that wasn't the problem.

I've read that this can sometimes be the coil. Here's a picture of my distributor:



I don't think this is the standard distributor that comes with the 4.1L engine. It appears to have an integrated coil.

Is there a good test procedure that will tell me if the coil is the problem? If so, where might I find this distributor part?

Open to any other suggestions too. I'm pretty stumped. Thanks!

[Andy4639]

First thing is check the wire to the cap for the coil. That is a later model HEI distributor it needs 12 volts all the time to work properly. These truck came with a resistor wire in the harness to cut down the voltage to 9 volts at start and while running. If it's the factory wiring hooked to it then that is your main problem.

[trevarthan]

Originally Posted by Andy4639

First thing is check the wire to the cap for the coil. That is a later model HEI distributor it needs 12 volts all the time to work properly. These truck came with a resistor wire in the harness to cut down the voltage to 9 volts at start and while running. If it's the factory wiring hooked to it then that is your main problem.

I'm seeing between 9 and 10v while running at the blue wire on the distributor. At the battery I'm seeing between 14.3 and 14.5v while running.

I'm seeing full battery voltage (12.84v) at the blue wire with the engine stopped but the key in the ON position.

This truck drove fine for 3 years with the same components (PO installed them all). It only recently started exhibiting this intermittent power loss behavior. Why would this just now become a problem?

[68 P.O.S.]

Here's a great thread on changing out the power wire by dmjlambert: http://67-72chevytrucks.com/vboard/s...d.php?t=708975

What is your timing set at?

[trevarthan]

ah, I'm seeing it's a resistance wire, not a ballast resistor.

[trevarthan]

Hmm. Not sure what's going on here.

Yellow wire still attached to starter (that may be an old wasp nest above the starter):



Yellow wire spliced into blue wire, which goes to HEI distributor:



However, fuse block appears to have a pink wire already, instead of the expected white resistance wire:



slightly better angle (definitely pink wire):



Confused. Why am I seeing 9v at the yellow/blue splice with the engine running if the white resistance wire has already been replaced with a pink wire?

[trevarthan]

Ok, I researched this HEI distributor the best I could. I read the coil ohms with a meter and saw less than 1 ohm. Couldn't find the exact specs on my particular coil but I figure that's probably a within-spec reading.

As far as I can tell everything is grounded fine. Took the cap off and ran it. No visible sparks.

Reached out and touched the power wire and it's noticeably warm. Voltage seems to drop from 10v to 9.1v as the engine warms up. It's pink, not white, but it still seems to be acting like a resistor, so I think I'm just going to order a replacement. It may not be my problem, but it seems like it's *a* problem anyway.

[VetteVet]

Originally Posted by trevarthan

Ok, I researched this HEI distributor the best I could. I read the coil ohms with a meter and saw less than 1 ohm. Couldn't find the exact specs on my particular coil but I figure that's probably a within-spec reading.

As far as I can tell everything is grounded fine. Took the cap off and ran it. No visible sparks.

Reached out and touched the power wire and it's noticeably warm. Voltage seems to drop from 10v to 9.1v as the engine warms up. It's pink, not white, but it still seems to be acting like a resistor, so I think I'm just going to order a replacement. It may not be my problem, but it seems like it's *a* problem anyway.

Most of the guys on here that have converted to an HEI distributor will tell you that you need a positive 12 volts going to the distributor, to the BAT terminal, and also that you no longer need the yellow wire from the starter for anything. That means that you also don't need the white resistance wire from the firewall block. You can replace it with the red wire to the distributor or you can run a red wire from the fuse panel IGN-UNFUSED terminal to the Bat terminal on the distributor. Either way it gives you 12 volts key on ignition to the distributor. You will have to have that before you can proceed with any trouble shooting. It may fix the problem or not but, you must have that.

From your description it sounds like your ignition is breaking down due to the low voltage and the way the wiring is set up would explain that.

[68 P.O.S.]

What is your timing set at?

[trevarthan]

Originally Posted by 68 P.O.S.

What is your timing set at?

I don't know, but I've ordered a timing light. Should be here in a few days. I used to own one, but it was in another life. I need this to confirm the vacuum advance is operating correctly and that it hasn't skipped a tooth on the timing gear, right?

[Grumpy old man]

You posted this a few years ago --

Hello,

I bought this truck a few months ago. Sometimes, when I turn the key off, the engine spins down, then "coughs" and tries to turn over one last time. The previous owner thought this was a malfunctioning carb, so the truck came with a carb rebuild kit.

I've since replaced the door locks, axle bearings, brakes, turn signal switch, fuel pump, power steering pump, radiator cap, and even the air filter. I think it's pretty funny that he prioritized this minor issue as a must-fix-first thing, but he's an old school engine racing guy, so I guess that makes sense.

Is this a carb issue? Will a rebuild kit fix it? Is there a book I could buy that would teach me how to install the rebuild kit? I replaced a head gasket once on a 94 toyota pickup, so I'm pretty sure I'm handy enough to handle it. I just need instructions.

Alternatively, Autozone sells an automatic choke carb for about $250. I assume this is an auto choke carb since I don't see a control anywhere for the choke, and under the air filter there was a little doo-hickey labeled "AC". It must not be working though, because I have to gas it for 30 seconds when I start it to keep it running.

Any info appreciated. I'm having a lot of fun bringing the mechanicals on this thing back to working order.

Thanks!


Did you ever rebuild the carb ? I don't see a choke cable going to the carb ? did you install an auto choke ? have you checked the fuel float level ? have you checked the heat riser valve below the exhaust manifold to see if it's turning freely when cold and hot ? Take a picture of the passenger side of the carb . The previous owner told you it had a carb issue and gave you a rebuild kit did you fix it ? What color smoke comes from the exhaust ? When you check the oil level does the oil on the dip stick smell like gas ?

I don't think you have an ignition problem ,I think you have a fuel issue from improper float / choke issue , possible stuck heat riser.

[trevarthan]

Originally Posted by Grumpy old man

Did you ever rebuild the carb ? I don't see a choke cable going to the carb ? did you install an auto choke ?

No, I haven't rebuilt the carb. My understanding is that it has an auto choke, yes. I didn't install it, but I've never seen a choke in the passenger compartment. I'll feel really, really dumb if it turns out it had a manual choke all this time. :p

We were thinking this device may be the auto choke:

Originally Posted by Grumpy old man

have you checked the fuel float level ?

Doing a quick google search, this appears to be something you can only do when rebuilding a carb, right? so no, I've never done this.

Originally Posted by Grumpy old man

have you checked the heat riser valve below the exhaust manifold to see if it's turning freely when cold and hot ?

No, and I'm not seeing much info online about that. How do I do that?

Originally Posted by Grumpy old man

Take a picture of the passenger side of the carb . The previous owner told you it had a carb issue and gave you a rebuild kit did you fix it ?

Ha. Wait... what? Are you the previous owner, or did you just read my post history? Looks like I posted about that rebuild kit back in 2014. I had completely forgotten about that.

I do remember I had a gasket kit in my closet until a few months ago. I tossed it because I couldn't remember what it was for and thought it might have been left over from my old chevy impala, which I sold years ago. That was probably the carb rebuild kit.

I do remember the previous owner telling me he thought something was wrong with the carb, but it always ran fine, so I didn't worry too much about it. I just replaced my first small engine carb a few months ago. I've never rebuilt one. I'm pretty ignorant when it comes to carbs.

Looking back at that old 2014 thread, it looks like I was complaining about the engine dieseling after I shut it off. I never did buy a timing light to diagnose that issue. I have one on order now, so I'll check that out when it arrives.

Originally Posted by Grumpy old man

What color smoke comes from the exhaust ?

Same color it's always been. Smokes on startup pretty fiercely then stops once it's warm. I think it's a blueish color.

I had a bad head gasket on a toyota once, decades ago, so I keep an eye on the fluid levels just in case it's a head gasket issue. The radiator does eat some coolant, but the radiator itself had a pretty bad leak when I bought the vehicle and I had it patched by the local radiator shop. That helped, but I think it still leaks a bit, so I never worry about it too much and just keep an eye on the coolant. If it is a head gasket and it's burning coolant, it must not be burning much because it takes a long time for the coolant level to drop. I always just figured it was burning oil from a leaky seal somewhere.

Originally Posted by Grumpy old man

When you check the oil level does the oil on the dip stick smell like gas ?

I hadn't thought to sniff it before, but you're right. It does smell faintly of gas. I read that tends to indicate a fuel pump leak, yes?

Originally Posted by Grumpy old man

I don't think you have an ignition problem ,I think you have a fuel issue from improper float / choke issue , possible stuck heat riser.

Hmm. Well, I'm happy to investigate that possibility. Tell me what to do.

In the mean time, I'm planning to replace the ignition switch. It's a $15 part and I just remembered last night that my old chevy impala did this same thing and the dealership changed the ignition switch to solve the problem. Seems worth a try.

[Grumpy old man]

Yes , I went back and read your old posts to see what you have been dealing with or changed lately , It makes it easier to see what the problem your having might be .

Automatic chokes need to be adjusted correctly , The choke ( top flap on carb) should be closed when engine is cold in the morning and open fully once engine has warmed up . AUTOMATIC DOESN'T MEAN FOREVER Find a real mechanic to rebuild that carb and have them adjust it afterwards , Don't waste your money on an ignition switch ,That's not your problem . When the previous owner hands you a rebuild kit and tells you it needs to be done , DO IT !

Pull 1-2 spark plugs and post a picture of the end so we can see what the plugs look like

None of this below is more important than your carb working correctly .

(I've since replaced the door locks, axle bearings, brakes, turn signal switch, fuel pump, power steering pump, radiator cap, and even the air filter. I think it's pretty funny that he prioritized this minor issue as a must-fix-first thing, but he's an old school engine racing guy, so I guess that makes sense.)

If it was mine I'd
pull the entire intake /exhaust manifolds clean and re gasket
Change oil
new plugs
rebuild carb

There is a difference between getting it to run and getting it to run correctly . These old engines/trucks require constant maintenance . there are a number of members in the Chattanooga area find one to help .

[RichardJ]

OK, now that you guys are done with your silliness over your "full 12 volts".

>>The truck idles, but loses power when warm.<<

I need a little more description about loosing power. You have a manual trans. Will the engine not rev above idle or is the problem occurring when you engage the clutch and it just won't pull?

Probably not related, but explain the "T" fitting on the PCV Valve. One hose probably goes to the Vacuum Port on the manifold next to the carb base. What does the other hose go to?
Have you checked for vacuum leaks, especially in the manifold to head gasket.

When you replaced the power wire to the distributor, did you remove the yellow wire from the "R" terminal of the starter solenoid? The two coils inside the solenoid can generate a very high, back-EMF voltage spike. The internal, "R" contact arm should not be contacting when the EMF spike is generated, but if it is the HEI module will be destroyed. This voltage spike would have had no effect on the old points ignition.

[toolboxchev]

Focus on ignition first, then fuel. An HEI Module can run for a while when bad, then when heat is applied it will die out.

Try a cheapo 20 dollar replacement. It is hard to diagnose this particular part.

[HO455]

Everything you ever wanted to know about HEI's and some you didn't.

http://garage.grumpysperformance.com...gnitions.2798/

[trevarthan]

Originally Posted by toolboxchev

Focus on ignition first, then fuel. An HEI Module can run for a while when bad, then when heat is applied it will die out.

Try a cheapo 20 dollar replacement. It is hard to diagnose this particular part.

Done. Module replaced. Problem persists.
Posted via Mobile Device

[67 chevelle]

Try driving with the fuel fill cap off , if that fixes it it is vapor locking

[trevarthan]

Originally Posted by RichardJ

OK, now that you guys are done with your silliness over your "full 12 volts".

>>The truck idles, but loses power when warm.<<

I need a little more description about loosing power. You have a manual trans. Will the engine not rev above idle or is the problem occurring when you engage the clutch and it just won't pull?

The problem is that the engine won't pull with the clutch engaged.

Originally Posted by RichardJ

Probably not related, but explain the "T" fitting on the PCV Valve. One hose probably goes to the Vacuum Port on the manifold next to the carb base. What does the other hose go to?

Looks like a charcoal canister near the battery:

Originally Posted by RichardJ

Have you checked for vacuum leaks, especially in the manifold to head gasket.

Just sprayed carb cleaner all around the carb. Absolutely no change of idle. Even sprayed it directly into the air cleaner. Still no change. That last bit is odd.

Originally Posted by RichardJ

When you replaced the power wire to the distributor, did you remove the yellow wire from the "R" terminal of the starter solenoid? The two coils inside the solenoid can generate a very high, back-EMF voltage spike. The internal, "R" contact arm should not be contacting when the EMF spike is generated, but if it is the HEI module will be destroyed. This voltage spike would have had no effect on the old points ignition.

I left the wires attached as the PO had them attached and I didn't feel like removing them yet. The old wire is still in the harness but is no longer attached to 12v ignition power or the distributor.

Here's what it looks like now with the new ignition wire:





UPDATE: we also just replaced the distributor cap and coil, just in case. no change. still stalling under power. We did learn that this is apparently an HEI distributor from a 1979 model though.

[JDL]

Check fuel presser and filter sock in the tank

[trevarthan]

Originally Posted by JDL

Check fuel presser and filter sock in the tank

When we had the b-nut off on the carb we tested to see if it was delivering fuel. Seems to be. Filled a container pretty quickly while turning the engine over.

I'm not sure how to do a more in-depth fuel pressure test. Which gauge would I need and where would I get it?

My understanding is that the filter sock is inside the fuel tank and the fuel tank is kind of a pain in the ass to remove. I'm probably more likely to replace the fuel pump first and work my way back.

I've also read the ignition switch can be a problem. They're cheap and look pretty simple to replace.

Problem is that I'm still not sure if it's an electrical problem or a fuel problem. I'm just replacing parts blindly here hoping for the best. Kind of a crappy way to debug a problem, in my opinion.

[RichardJ]

The carb may need rebuilt, but before I make that suggestion I need a better understanding of the existing vacuum line connections.

In my first post I ask about the "T" on the PCV vacuum lines. From the first picture above, I thought B-A was a single line that looped across the valve cover and back to the PCV "T". I now see that A and B both go across to the pass fender and to the Charcoal canister.
My question was and still is, what is the vacuum line on the driver side of the "T" and what is it connected to?

The large casting port on the carb air horn usually would have a fitting that would connect a hose to the charcoal canister. Sometimes the casting is there, but not drilled out.

The open port directly below the fuel inlet is probably for the air cleaner Thermac door. Just checking.

I see a vacuum line connected to the distributor vacuum advance, but I can't see it on the other side, connecting to the carb? There is a steel line laying against the valve cover, but would still like to see where it is connected.

[trevarthan]

Originally Posted by RichardJ

The carb may need rebuilt, but before I make that suggestion I need a better understanding of the existing vacuum line connections.

In my first post I ask about the "T" on the PCV vacuum lines. From the first picture above, I thought B-A was a single line that looped across the valve cover and back to the PCV "T". I now see that A and B both go across to the pass fender and to the Charcoal canister.
My question was and still is, what is the vacuum line on the driver side of the "T" and what is it connected to?

I'm happy to collect photos and video of this vehicle.

The vacuum line on the driver side of the "T" Looks like it is connected to a port in the intake manifold:

Originally Posted by RichardJ

The large casting port on the carb air horn usually would have a fitting that would connect a hose to the charcoal canister. Sometimes the casting is there, but not drilled out.

The open port directly below the fuel inlet is probably for the air cleaner Thermac door. Just checking.

There are two vacuum lines on the air horn. One goes to a pancake on the inlet:



The other goes to that port on the carb below the b-nut:



Both go to a sensor of some kind on the air cleaner assembly.

top of air cleaner, open and removed:



bottom of the air cleaner:



That sensor is labeled AC (auto choke?) and looks like it has a port for an electrical connector (inside the air cleaner), but it's never had anything plugged in there for as long as I've owned it. No clue what that is. Let me know if you know. I'm curious.

Originally Posted by RichardJ

I see a vacuum line connected to the distributor vacuum advance, but I can't see it on the other side, connecting to the carb? There is a steel line laying against the valve cover, but would still like to see where it is connected.

It is connected to the carb here:



I've tested the vacuum advance on the distributor with a rented hand held vacuum pump. Seems to be working (saw the lever move), but I don't know if it's within spec or not because I don't know the specs or where to find them.

EDIT: I just noticed you asked about that sloped port on the carb. It does appear to be drilled to me, according to this photo with the cleaner removed:



I took a video of the auto choke opening for the previous poster. At the end of the video I pan around the carb a bit for you. This link should take you right to that point in the video (turn your sound down - loud):
https://youtu.be/94MVneCeIDE?t=473

[Average Joe]

I dont mean to throw you a curve ball here, but I had a similar situation in my last truck. It would stumble randomly. In the beginning it appeared to be doing it under load. I replaced fuel filter and pump to no avail. I then rebuilt the carb. Still happened sporadically. IIRC I tried a known good coil. Still no luck. Eventually I traced the problem to a crappy splice in the main power wire coming from the battery, via the small distribution block on the passenger fender, to the fuse box. I soldered and shrink wrapped the faulty splice and was back in business.

Years later I had a similar problem in my Suburban. I traced it back to a loose bulkhead on the backside of the fusebox.

[trevarthan]

Originally Posted by Average Joe

I dont mean to throw you a curve ball here, but I had a similar situation in my last truck. It would stumble randomly. In the beginning it appeared to be doing it under load. I replaced fuel filter and pump to no avail. I then rebuilt the carb. Still happened sporadically. IIRC I tried a known good coil. Still no luck. Eventually I traced the problem to a crappy splice in the main power wire coming from the battery, via the small distribution block on the passenger fender, to the fuse box. I soldered and shrink wrapped the faulty splice and was back in business.

Years later I had a similar problem in my Suburban. I traced it back to a loose bulkhead on the backside of the fusebox.

Nah, it's fine. I've got 50/50 money on fuel or electric at this point.

How did you determine it was a bad splice? Jiggle the cables? I've been doing that and I haven't noticed any odd behavior. I may still not be jiggling the right one though.