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Old 07-12-2007, 03:04 PM   #1
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Default Overheating | Bad Idle | Backfiring = the suck

History:
New [to me] shortblock
reused head
new headgasket
timing belt
waterpump gasket
thermostat gasket

Timing = dead on
Compression = 187 | 183 | 181 | 185 (might have forgot WOT)

So I got my engine all squared away a few days ago and got brave enough to take it for a drive.

Smooth, no leaks, drove for about 15-20 miles around town.

Decided to drive it home on the highway, 30mile drive.

No problems, sustained 70mph, no signs of problems....datalog looks great.

Just as I'm about to pull off the highway I notice my legs are getting warm, stop at the stoplight and smell coolant...my radiator cap wasn't seated well and was dripping fluid. Temp gauge on MS = 173F (actually 203) fans kicked on as I fiddled with the cap.

Get back in the car and the idle is very lump, almost 3-cly like. Start to drive once I pull back in line at the next green light...car has no power, feels like 3-cly. Then "click", perfect again. Like an on/off switch, no power. Power back.

Drive to the next stoplight, lumpy idle again....green light, give it load, good again. Notice 17~Hg. (40kPa) of vacuum at idle. about 3~hg. low. (usually at 32kPa).

Park the car.

The day before I had trouble with the gasket on my IAC valve, so I decided to block the lines, incase somehow a small amount of coolant was injecting into the intake at idle. No dice.

Left it alone. Next day I got to drive it another 30miles back to the garage I work on it at. Started up perfectly, 3-cly syndrome again. Get on highway, overheats almost instantly. 230F. Pull over. Thought about it in my head and considered a jumped tooth on the timing belt. Try to take off the valve cover and whatnot, but I have no luck in rotating the crank. Roadside assistance shows up and I put everything back together. Check my coolant, seems a tad low (but overflow is higher than usual). Refill the radiator.

Drive off. Starts to overheat within 2-3miles. Drive with heater on, sustained 220F for the trip home in 87 weather (not fun BTW).

I suspect the timing belt at this point, however still looks dead on. 19 teeth between the marks at TDC. notice a little slack, so remove and redo, tension better.

Recheck the timing, had to adjust. makes me think it was off.

In the meantime I check the thermostat in boiling water. Opens at 180 as it should.

Burp the system. let it idle as I go through my software.

Idle gets lumpy vacuum low. Unplug the IAC and idle is normal again, 1020RPM, but at least smooth.

So I decide to take it around the block to see how it reacts. Coolant stays high 220F, drives perfectly.

Get into a bit of boost to see how it reacts and backfires out of the exhaust.

Go back, check compression: 170 | 168 | 161 | 170

only thing I can think of at this point is the head and gasket??? at a loss.....help

I either got three problems or one really large one....not quite sure whats up. Idle valve had always been working prior, overheating sucks, and the backfire doesnt make sense. Unless for some reason I didnt have the crank at TDC when I aligned the cams.


log of backfire: https://www.miataturbo.net/braineack...e/backfire.jpg
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Old 07-12-2007, 03:09 PM   #2
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Bad waterpump? The engine already had a blown head gasket right? Issue that caused it could still exist on the engine.
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Old 07-12-2007, 03:12 PM   #3
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No idea about the on-off power, but the rest of it screams headgasket. Was your reused head checked for warpage?
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Old 07-12-2007, 03:16 PM   #4
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used my good (2yo <10k) waterpump. I didnt check my head. Just took it off in 5 steps or so, and put it on via the service manuals proceedure, about 5 steps of torque again.
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Old 07-12-2007, 03:17 PM   #5
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I don't beleive your RPM readings from the datalog capture. No way you had a dozen oscillations from 0-7000 rpm in 4 seconds. Not mechanically possible.

Suggest you pursue what drives the MS's RPM signal.

Otherwise, data suggests headgasket. However, I am not yet convinced that there is not an electrical problem.
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Old 07-12-2007, 03:22 PM   #6
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after seeing that datalog I'd check your wiring. what ben said.
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Old 07-12-2007, 03:22 PM   #7
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I know the info.... posted is good....

However, since this happened post "test drive".... what about a loose ground? Is it possible that something else worked it's way loose?

Check the connectors, check the grounds, if it's nothin' other than to elminate them. <G>

Dave,
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Old 07-12-2007, 03:27 PM   #8
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Quote:
Originally Posted by Ben View Post
I don't beleive your RPM readings from the datalog capture. No way you had a dozen oscillations from 0-7000 rpm in 4 seconds. Not mechanically possible.

Suggest you pursue what drives the MS's RPM signal.

Otherwise, data suggests headgasket. However, I am not yet convinced that there is not an electrical problem.

acutally, the rpm spikes to 106007.0 I had to limit the scale.

On the gauge itself it looked and felt just like a rev limiter.

Would you suggest I warped my head on removal, didn't torque it good and warped it on install, not torqued, or somehow the block was warped?


Doesn't seem to have coolant in oil or vise vera.
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Old 07-12-2007, 03:30 PM   #9
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I was always told when you remove the head even off a healthy motor that you should always get it machined before reinstalling. Cheap insurance I suppose.
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Old 07-12-2007, 03:33 PM   #10
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I suppose I can pull it off easy enough and check the gasket and head. What's pointing to the headgasket?
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Old 07-12-2007, 03:35 PM   #11
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Quote:
acutally, the rpm spikes to 106007.0 I had to limit the scale
Ignitor. I had spikes to 100k rpm and thus fuel cuts when my ignitor failed.

I wondered why your numbers were so round.
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Old 07-12-2007, 03:36 PM   #12
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Quote:
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Ignitor. I had spikes to 100k rpm and thus fuel cuts when my ignitor failed.

I wondered why your numbers were so round.

But would that explain the over heating?
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Old 07-12-2007, 03:41 PM   #13
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so do i have like 17 different things going wrong now? and it all happened at once during one drive?
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Old 07-12-2007, 03:47 PM   #14
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here's the complete log of the drive home yesterday, this was where everything seem good. Finally, at the end is when I pulled to the first stoplight and the idle was lumpy as described.

https://www.miataturbo.net/braineack...rebuild_66.xls

and a short one of today:

https://www.miataturbo.net/braineack...verheating.xls
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Old 07-12-2007, 03:52 PM   #15
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are you sure you installed the gaskets on the intake manifold correctly? backwords, although it should leak, would block coolant going into the head, and warp it during the over heat.

I second checking grounds and any possible vacuum leaks
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Old 07-12-2007, 03:55 PM   #16
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looking at my current log, it's almost as if the manifold temp is going up with the coolant...that's usually not the case.


I'm pretty sure I put it in correctly, only one way to keep all passages open and for it to fit flush. It didn't overheat untill the next day. I drove over an hour on it prior without any signs of overheating.

only three ground I remember pulling, and they are still there, good and tight. two near the back of the fuel rail and the one by the heater hoses.
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Old 07-12-2007, 03:57 PM   #17
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did you check the gasket? I put my intake gasket in backwards once, and like I said it leaks, but it could cause the problems. perhaps in upside down and backwards? I dunno.
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Old 07-12-2007, 04:03 PM   #18
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kept intake manifold on. guess the only way to tell at this point is a leakdown test (dont have the gauges) or pull the head off again and see.

I guess we are assuming that coolant is leaking into the combustion chamber or onto the block? There are absolutely no signs of leaks or loss of coolant or smoke.
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Old 07-12-2007, 04:11 PM   #19
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duty cycle 125%?

maybe your map is F'd up. reload?
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Old 07-12-2007, 04:13 PM   #20
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heres a thought - is the water really getting that hot - ie boiling ?and making all the associated noises ? perhaps it is all an electrical problem ?
pinched wire somewhere ?
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