Reply With QuoteYou need to be WOT when doing a compression test. Without that, the test is invalid.
A leakdown is a better test.
Did you measure the resistance on the plug wires? They do burn up frequently.
Fuel injectors dirty? Those can be rotated too.
Engine Woes
When I backed my car off the trailer after Hallett, I noticed a rough idle. I pulled one plug wire at a time and there was no change when I pulled the wire off number 4.
The plug was dirty but firing, so I did a compression test. Warm engine, pulled CAS wire, removed all sparking plugs, spun until highest number. But, I forgot to go WFO on the throttle. The numbers are; 135, 128, 85, 85. Would opening the throttle make the number look better or worse? Engine is a 1994 with about 135,000 miles. Even if the head gasket is blown between 3 and 4, I think the 1 and 2 numbers still mean an overhaul. And, I still don't know why number 4 is not firing.
The rough idle is not so noticeable when the engine is hot. At Hallett, I did notice the the idle was rough when I was running the A/C in the grid before the Sat. afternoon sessions. I'm not sure if I should be kicking myself for not being more concerned about the rough idle, or proud that I was running in the low 1:39's with this sick old tired engine.
Anyone have an NB2 engine for sale?
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
You need to be WOT when doing a compression test. Without that, the test is invalid.
A leakdown is a better test.
Did you measure the resistance on the plug wires? They do burn up frequently.
Fuel injectors dirty? Those can be rotated too.
If the throttle was closed during the entire test it is not invalid as far as comparing cylinders..........going WFO isn't going to reveal anything more than he already knows.
Yea those numbers are already low even if they were 135 Accross....I'd do a leak down to make it forsure and then just rebuild that motor in ur garage they are super easy to rebuild, and maybe throw some high comp pistons and cams in it and make 150rwhp...
I'll redo the compression test today, with WOT. And, try cleaning the fuel injectors. But, it's raining now and I have a couple of other pressing projects that need to be addressed first.
Thanks for the suggestions.
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
I re-did the compression test. Warm engine, pulled CAS wire, removed all sparking plugs, WOT, spun until highest number. The numbers last time were; 135, 128, 85, 85. This time with WOT they were, not much changed, 135, 128, 125, 85. Number 3 changed from 85 yesterday to 125 today. I did that cylinder twice each day and got the same reading both times, each day.
Plug wire resistance is 3.5k, 2.8k, 2.3k and 1.9k ohms, 1 to 4. I'm thinking they are good.
The resistance for the number 4 injector is 14.5 ohms. I listened and can hear all four injectors clicking.
Next, I will remove the injectors and make sure number 4 is spraying. But, that may not be until Monday.
The part for my small tractor came in today, so I got it back together and out of the shop. Now, the Miata is in the shop.
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
Might try a wet compression test (i.e., a bit of oil in the cylinder). See if that brings up #4. Will tell you if it's piston ring related vs. head/valve related.
Any sign of coolant on the plugs or in the cylinder? With a head gasket issue, you'll almost always have coolant going somewhere.
Have you taken off the valve cover to look at the camshafts? Might have wiped a lobe.
Trying to think of easy things first, because we're quickly getting to the pull head/pull engine stage.
No signs of coolant on the plugs or dip stick.
Today, I removed the injectors and cleaned them. They seem to be working fine. Except, the fuel comes out in a stream, I expected a fog. FYI, those little black plastic spacers are hard to find when dropped. After re-installation, nothing changed, still, number 4 is not hitting and the idle is rough.
My plan for tomorrow is to remove the cam cover and maybe do the "wet" compression test.
At times like this I sure do miss carburetors and points.
I'm trying to learn as much as I can from this situation, I am still planning an VVT, MS3 swap.
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
I think I am becoming a pro at this compression testing. I did a base line again today; 135, 122, 128, 85. I found an SAE article that said to add 2 oz. of oil for a wet test, so I did. Two oz. seems like a lot of oil, so after the first cylinder, I cut the oil down to a little over an ounce. Reading were; 250, 250, 220, 210. This makes it look like bad rings. But, I still had the misfire.
I had some Sea Foam, so I gave it a try. I used a whole can vacuumed into the engine. The engine seemed to be running better. So, I when out and drove the ever loving Sea Foam out of the car.
The miss was gone. After the Sea Foam, I did yet another compression test. This time the reading were; 130, 127, 122, 110. And all cylinders firing.
I killed some other rats, then came back to the Miata after a couple of hours. The miss is back.
Tomorrow, I'm going to town and buying a BFH.
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
Time to take it to Rogue or TDR!
1990 White NA - SOLD
1994 Black NA - SOLD
2006 Red NC - GT with limited slip, HIDs, all OEM.
Did you test the coil pack?
When I pull the plug wires off the plugs, I can hear the spark jumping to the plugs. I don't know how to bench test the coils to see if they make a good strong spark to the plugs.
If I pull the wire far enough on 1, 2, and 3, the engine runs even rougher. When I do 4, removing the wire does not change the roughness of the engine, but I can hear the spark for a short distance.
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
Wouldn't low compression cause a miss?
http://www.oreillyauto.com/site/app/...icle=800649141
Last edited by POS Racing; 09-25-2013 at 09:28 PM.
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