3 replies to this topic

[Miatawithproblems]

My 1999 Mazda Miata is having some problems I can't seem to diagnose....


It is real rough to start, it will crank hard and strong forever, but won't start unless I push the throttle in and out a couple times at the right moments. Then after it starts, it will idle correctly then after a while it will drop to 400 revs and if I don't touch the throttle it will stall. If I drive it around, and let off the gas, the revs will most of the time drop to 400 and stall. However.... if I keep on the gas, and do a steady speed, the car runs like normal. If there's any drop in performance it's extremely slight, I can't tell a difference.


I was running the heater everywhere I went and managed to do 100 miles on the car at night and the car held steady. Any time i turned the AC on it would overheat after 30 seconds.

I took the car into a cheap mechanic and he found the heater core hose was blown, he rerouted the heater core.... and still it has the same problems. I have not used the A/C though since this change.



Thoughts? Does this sound like a blown head gasket, a lot of people have been saying this is not the case?


After looking around the options I've found could potentially be:

Head Gasket
Catalytic converter clogged
Crankshaft sensor
Camshaft position sensor
Main Relay


The problem is.... why would it overheat if it was anything except the head gasket? But if it were a head gasket.... wouldn't there be more problems?

[exninja]

I think you're confusing cause/effect of the head gasket and overheating issues. Many times, an overheated engine will cause your head to warp which causes a head gasket failure. So a blown head gasket can be a sign that you've overheated and your cylinder head is no longer flat. But a blown head gasket won't necessarily cause your engine to overheat. There are a few signs of a blown head gasket: milky oil, oily coolant, or sweet, white smoke in your exhaust. Each one just depends on how the gasket blew. Your issues sound more like an idle control issue or TPS issue (I'm new to Miatas). The A/C thing may or may not be related to the idle/starting issue. I've seen a lot of general Miata stuff on miata.net that might help with this.

[KentCarter]

Do the cooling fans run when the a/c is on? Are they turning the right direction?
Is the thermostat ok?
Is the radiator clogged or dirty?
Fuel filter clogged?

[DES4]

First off, I'd consider that your cooling system might be marginal, and unless you are certain that it performs as it should, it might be a secondary issue here.

-Head gasket: Possibly. Pull your plugs and inspect them. Whatever they look like, it should be uniform across all four, any deviation MIGHT indicate a problem in that cylinder. Depending on the severity of a leaking head gasket, they can be either spotlessly clean or have a white buidup on them. If you believe it might be the head gasket, do a compression test. There is also a litmus dye test for the presence of CO in the coolant; it's pretty conclusive (at least as far as indicating that you are somehow getting Carbon Monoxide in the radiator tank). I've seen an emission sniffer used for the same, as well.

-Plugged catalytic convertor: Doubt it. Generally won't cause running or overheating issues, the most obvious sign of a plugged cat is loss of power/failure to rev.

-Crankshaft sensor: Again, nope. It'll cause either phantom misfire counts, an intermittant miss/stunble or a no-start. Overheating? No.

-Camshaft sensor: Couldn't imagine overheating from this one, either. You'd expect running issues certainly, but depending on what the limp/fail strategy is in the map, you'd likely get an obvious sign of something amiss, as well as a MIL displayed.

-Main relay: If you mean fan relay, it's a likely cause of an overheating issue, but it should only manifest itself at idle. A/C not blowing cold/high compressor head pressures would be indicators as well. Running issues, such as an idle drop-off/stall; doubt it. Main power or engine management relay will likely not cause anything but inoperative components or engine shutting off altogether.

Based on your first symptom, I'd suspect a vacuum leak or some other source of unmetered air entering the engine. EGR valve is always suspect. Would not explain the overheating, though...