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Old 08-29-2016, 02:21 PM   #1 (permalink)
jmg
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cooling help-- 350 xj

having a cooling issue that's driving me batty. I'm running a circa 1988 chevy 350, mild cam, efi, stock heads, novak aluminum rad w/ 3 spal e fans in my 88 XJ. Everything was working fine earlier this year -- rock crawling at rausche creek, zero overheating, driving at normal speeds on the highway, no issue. I grenade my d35 over the summer, replaced that with a f8.8, and since then, have had nothing but headaches with overheating. I'm sure one has nothing to do with the other --- only mention it because it was sitting around for a few weeks in a shop, not being driven. The day I get it back, it overheats on the drive home. This is driving me crazy. At idle, it sheds heat with no problem. But if I go over 2200 rpm or so, regardless of how fast I'm going, heat starts slowly climbing. At rausche creek two weeks ago, no issues with low idle crawling, but heading back on a long uphill access road, hit 230 before I had to pull over.
I'm at the point where I think I may have a blown head gasket (I'm going to do a compression test when I get back this week), but anything else on the "idiot" checklist I missed? This is what I've done:
  • confirmed tstat was working properly and in the right orientation. tried a 160 degree tstat, no difference. Tried running with no tstat, no difference
  • lower rad hose looked snarky, thought maybe it was collapsing. replaced, no difference.
  • checked timing -- running 12 degrees initial timing, 34 degrees total timing (timing was retarded --26 degrees total timing). no difference
  • Replaced water pump. Tried a stock replacement from advanced auto and a high-flow from edlebrock. Confirmed both were reverse rotation pumps. No difference.
  • Raised a/f ratio at cruise to 13.8. No difference.
  • changed icm (I know, but I figured what the hell)
  • confirmed all fans are working. Set them to turn on at 180.
  • changed tensioner and serpentine belt (it was late and I ran out of ideas)
  • pulled winch, lights, and everything else from the front of the truck. Thought this may be impeding air flow. No difference.

Is there anything -- other than compression test-- that I've missed?
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'88 XJ|| SBC 350 - zz6|| Other stuff that keeps breaking

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Old 09-01-2016, 09:28 AM   #2 (permalink)
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The only explanation for this is a blockage in the coolant system or a bad water pump. Simple as that. A car that stays cool at idle and overheats while driving is saying that there is a bottleneck in the system. Enough flow to cool at idle and not enough to cool when the motor is spun up and generating more heat.

You say you've replaced the water pump, so that leads me to believe that you have a clogged rad or a bubble in the system. My guess is that, when you had it in the shop to have the 8.8 swapped in, the jeep may have sat at a weird angle for a period of time. Air in the coolant system may have worked its way into a spot where it cannot burp itself from.

Try burping the coolant system. Sometimes this is tricky with SBC swaps as the rad may sit high or lower than the tstat. I fought the same battle. Google some tricks to fully burping an SBC. Check your rad hoses to make sure coolant is flowing freely and in high volume until you're sure you've got the air out of the system.

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Old 09-01-2016, 12:35 PM   #3 (permalink)
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The only explanation for this is a blockage in the coolant system or a bad water pump. Simple as that. A car that stays cool at idle and overheats while driving is saying that there is a bottleneck in the system. Enough flow to cool at idle and not enough to cool when the motor is spun up and generating more heat.

You say you've replaced the water pump, so that leads me to believe that you have a clogged rad or a bubble in the system. My guess is that, when you had it in the shop to have the 8.8 swapped in, the jeep may have sat at a weird angle for a period of time. Air in the coolant system may have worked its way into a spot where it cannot burp itself from.

Try burping the coolant system. Sometimes this is tricky with SBC swaps as the rad may sit high or lower than the tstat. I fought the same battle. Google some tricks to fully burping an SBC. Check your rad hoses to make sure coolant is flowing freely and in high volume until you're sure you've got the air out of the system.
Thanks. I burped it when I refilled it, but didn't think to try it again. I'll try that, see how it goes.

one of my friends has a purge system that is "guaranteed" to bleed coolant system and fill without any air. I may give that a try.
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Old 09-01-2016, 12:56 PM   #4 (permalink)
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does your volt meter check current? I could see that all 3 fans are pulling the right amount of power. 1 or 2 might be spinning but not at full capacity.
Does your heater blow hot air when the engine is hot? If not, that would lead back to the "air in the system" thought.
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Old 09-01-2016, 06:37 PM   #5 (permalink)
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does your volt meter check current? I could see that all 3 fans are pulling the right amount of power. 1 or 2 might be spinning but not at full capacity.
Does your heater blow hot air when the engine is hot? If not, that would lead back to the "air in the system" thought.
The fans only help the cooling system when at idle or low speed. So I doubt that is the source of the problem
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Old 09-01-2016, 07:19 PM   #6 (permalink)
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yeah, thanks for the suggestion on the fans-- I checked, they're drawing exactly the amps they're supposed to be drawing. It is shedding heat with zero problem at idle and stop-and-go traffic, so no surprise there, but appreciate the suggestion. Heater is blowing hot air.

I'm burping this thing several times tonight. If that doesn't work, I'll be back.
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Old 09-02-2016, 05:29 PM   #7 (permalink)
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The fans only help the cooling system when at idle or low speed. So I doubt that is the source of the problem
I would absolutely agree with that. My thought was that was "IF" the fans were not moving at full speed, while the vehicle is traveling, slow moving fans could impede the air flow through the radiators. Just a thought. However, as the OP stated, they are working correctly.
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Old 09-03-2016, 06:32 AM   #8 (permalink)
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I had a similar overheating problem in my 350-powered CJ5 - mine was caused by a lean condition at higher engine speeds. I replaced the fuel pump and adjusted the float levels in the carb which fixed it. You said that you increased the A/F ratio at cruise, I would confirm that you are holding proper fuel pressure and A/F ratio while driving.
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Old 09-03-2016, 07:47 AM   #9 (permalink)
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I had a similar overheating problem in my 350-powered CJ5 - mine was caused by a lean condition at higher engine speeds. I replaced the fuel pump and adjusted the float levels in the carb which fixed it. You said that you increased the A/F ratio at cruise, I would confirm that you are holding proper fuel pressure and A/F ratio while driving.
Thanks-- I'm running at 46 psi on fuel, right where I should be. fuel pump is brand new. the holley has a diagnostic screen that lets me monitor a bunch of things, including fuel --- doesn't dip below 45 while cruising.
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Old 09-05-2016, 11:54 AM   #10 (permalink)
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ok-- I took this out again for a drive, decided to play with the target AFR values.
Lowering target seemed to either have zero effect or increase the rate at which coolant temps increased. This is all somewhat subjective of course, since I'm trying to drive while monitoring temps and don't have a passenger to help :-)

I pulled over, and tried increasing the target AFRs-- I started with something I consider very high -- 16.5 cruise AFR and 14.5 WOT AFR (in holley parlance, these are *target* afrs, and are more or less adjusting the AFR curve. Actual AFR observed AFR values are typically going to be less than the targets). This had a significant impact -- driving at 65/2800 rpm in 4th gear, the coolant temp actually topped out at 210. At 16.7 cruise AFR and 14.8 WOT AFR,running at the same speed/rpm, the tuck actually dropped to 209, and vacillated between 209 and 210. At those setting, even if I stepped on it, uphill on the highway at 75/~3200 rpm, coolant temps hit 214, and stopped increasing.

the only issue I have with my "tests" is that the ambient conditions are very different from when I first noticed the overheating -- the first time I noticed the overheating issue, it was 90, sunny, and humid out. Today it was 76/partly cloudy/dry out. So I have no idea if I'll see similar behavior in hotter weather. But it still did overheat (just not as quickly) in similar weather to today-- I tried two days ago in 80/cloud/dry weather, and driving at the same speeds on the exact same route, the coolant temp hit 225, and there was no indication it had hit a ceiling.

So, ambient temps aside, I think the ARF is at least a significant contributor to the overheating issue. But, 17AFR at cruise and 15AFR at wot seems extremely high to me... is this high? If so, is there some other underlying issue I should consider?

This makes zero sense to me, btw... higher AFRs should be causing overheating, not alleviating it... right?
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Old 09-05-2016, 12:32 PM   #11 (permalink)
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Going back to the first post. Everything was fine until the axle swap (rear?). did the exhaust pipe get damaged? pinched?

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Old 09-05-2016, 02:19 PM   #12 (permalink)
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Going back to the first post. Everything was fine until the axle swap (rear?). did the exhaust pipe get damaged? pinched?
no, exhaust is good as new, I visually inspected everything before I got it.
As an update, Novak agreed to send me a new rad. I'm getting only a ~10degree differential using a laser temp gun at the inlet and outlet, it should be closer to 30 degrees. Whether the rad is *the* problem, one contributing factor, or not an issue, I'll soon find out.

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Old 09-05-2016, 07:53 PM   #13 (permalink)
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Stoichiometric ratio of regular gas is generally around 14.1:1 AFR - below 14.1 is considered "rich" while above that is "lean". "Fattening" or "richening" the mixture means decreasing the AFR while "leaning it out" means to increase the AFR. Suggest you bring your cruise AFR down (making it less lean) into the mid 15s and WOT in the upper 12s to low 13s and see what happens to your overall driveability and temps. How are you controlling timing?
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Old 09-06-2016, 11:21 AM   #14 (permalink)
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Stoichiometric ratio of regular gas is generally around 14.1:1 AFR - below 14.1 is considered "rich" while above that is "lean". "Fattening" or "richening" the mixture means decreasing the AFR while "leaning it out" means to increase the AFR. Suggest you bring your cruise AFR down (making it less lean) into the mid 15s and WOT in the upper 12s to low 13s and see what happens to your overall driveability and temps. How are you controlling timing?
I'm not sure how much of the "results" I saw over the weekend were the result of my playing around with AFR values or that it was 76 and dry out. I saw zero difference in drivability or idle quality no matter what I ddi to the target numbers, but increasing the target for cruise afr did seem to help on overheating. it just baffles me, increasing the AFR target value should make the overheating problem worse, if anything. I've bought a new wbo sensor just in case mine is faulty, haven't had a chance to put it in yet.

I'm running a mechanical large cap HEI. I originally had thought my timing was too retarded, but as stated in the OP, I'm running 35 degrees total timing. I was going to start monkeying around with the weights to work on the curve, but decided to just go with a small cap computer controlled distributor, which I just bought. It was something I had always planned on doing, just not at this point.

Been working with novak through these issues, they're going to send me a new rad to try. Hopefully that does the trick.
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