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Discussion Starter #1
Hi, I hope this is the right place to post this.

I have a 1998 Hyundai Accent 1.5L manual transmission, 140k miles. Engine codes P0136 and P0133. General problem is that under load the ignition timing reduces from its proper value (20-35 degrees) down to 6 to 12 degrees, which is ballpark for idle and results in a misfire. The misfire can be cleared by reducing throttle. Wide open throttle can be achieved without misfire if the throttle is advanced slowly (idle to WOT over about 40-60s). When operating at a steady throttle above about 1000 rpm the car performs perfectly.

The story is that in March the crankshaft position sensor failed, which resulted in a sudden stall and subsequent crank no start condition lasting for about 20 min. Then the car would behave normally and the cycle would repeat 30 min later. The ignition coil was also replaced at this time.

In June the camshaft position sensor failed, which resulted in excessive vibration. This vibration damaged the knock sensor connector and engine mounts, which were replaced. The misfire problem occurred immediately following the failure of the cam sensor. When the camshaft position sensor was replaced the car continued to vibrate for about 2 more miles before the computer figured out the new sensor and resolved the vibration. I am not sure if this is normal or not.

Parts that have been replaced this year.


Ignition | seller | comments

Crankshaft position sensor. Advance auto parts. Resolved crank no start condition.

Camshaft position sensor. Amazon (intermediate quality). Resolved vibration problem.

Spark plugs and wires. Autozone. Some improvement to misfire problem.

Knock sensor and connector. Amazon (cheap) and Autozone, respectively. Old knock sensor had pinging problems at moderate throttles after sitting for several minutes. Driving the car after going to the store for instance produced this pinging. New sensor has resolved this problem.

Ignition coil. Autozone. No difference

Engine coolant temperature sensor. Advance auto parts. No difference

Acceleration sensor. Advance auto parts. Some improvement to idle. The idle is a bit rough when warm but does not stall.

Throttle position sensor. Amazon (cheap). No difference

Battery. Autozone. No difference. Original battery was 7 years old and had been drained twice.

Air | seller | comments

Air filter. Amazon. No difference, but none expected.

MAF. Amazon (really cheap). No difference. The original still works, I just wanted to make sure there was no dead spots at higher throttles, which are difficult to test without load. Assumption is that the original and replacement parts will not be defective in exactly the same way.

Idle air control valve. Advance auto parts. Slightly lower idle rpm.

Most vacuum lines, the others were inspected and no leaks were found. Walmart. No difference. The brake booster does lose vacuum about 3 seconds after turning off the engine. However, it has been like this for years. The check valve built into the brake booster allows a small amount of air into the intake, but none going the other way into the booster. There is missing vacuum line on back of the intake manifold, which I have capped. All hoses are attached at both ends, which means that the vacuum hose was either removed before I got the car or it fell off without me noticing.

PCV valve. Amazon (decent quality). No difference.

Canister purge valve. Autozone. Car was having some difficulty starting after filling with gas. Car starts nicely now.

The car does not have an EGR valve. I checked the exhaust system and there are no lines/hoses coming off the system. Also, I checked the catt and it is clear. I was able to blow air through it without much resistance and the meshes are intact and shiny. Also, I was able to blow air through the rest of the exhaust system without any resistance.


Fuel | seller |Comments

Fuel filter. Autozone. No difference. The car has sat for a while and some rust particles were noted 2 years ago when it was previously changed. However, no particles were noted this time.

Fuel injectors. Amazon (good quality). No difference.

Fuel pressure regulator. No difference.


What else is there that could be causing this problem? The fuel pump is original, but this does not seem like a pump issue since I can go to full throttle, but it does takes a minute to get there. I am considering the possibility of getting a cam sensor from Autozone, but other than that I’m out of ideas.
 

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Discussion Starter #2
Update. Faulty ignition coil, even though it was replaced in March, seems to have been the problem. On inspection there was engine oil on the bottom of the coil even though there was no apparent leak from the valve cover.
 
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