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View Full Version : Please Help! Erratic engine speed above 5000rpm



cheyu
01-20-2014, 10:45 AM
I've just bought my 1993 na8 four months ago, and it has been problem after problem for me:

1) First, the engine had an issue while idling. Once the engine heats up it would idle very roughly and even stall sometimes.

This was fixed once I cleaned the throttle body and IAC valve, and also readjusted the TPS sensor.

2) Then I noticed that the engine's performance was very limited below 3000 and 4000 rpm. When in WOT, below 4000rpm, the engine would hesitate for a long time before kicking in. But once you get above 4000rpm, it would suddenly become very powerful. I realised this was probably due to the engine's management system changing from closed loop to open loop control.

3) In hope of fixing this problem, I replaced the entire throttle body + IAC valve + TPS sensor with a cheap second hand purchase, but was of no avail.

4) Fitted new spark plugs and a fuel filter. Also did not fix the problem.

5) Finally, I realised the oxygen sensor (my car only has one) was at fault, it did not produce a voltage even when heated. So I ordered a new one.

6) During the time of delivery I drove my car by unplugging the TPS to force my engine into open-loop control, so I can at least get the drivability I needed when going up ramps and bridges.

7) I Finally received the new O2 sensor today and had it replaced. The car ,now, drives very nicely. No hesitation below 4000rpm and pulls linearly. BUT, when the engine exceeds 5000rpm this happens:
http://www.youtube.com/watch?v=z7jqJgd0E1Q
(Basically, the engine speeds starts to jump all over the place between 5000- 7000rpm even if I keep my foot steady)

Could I have damaged the engine somehow by driving it with TPS unplugged for an extended period of time? I've only driven it three times while waiting for the arrival of my new O2 sensor, and each time not exceeding 15km in heavy traffic.

Please can someone give me any suggestion what might be causing this problem? It seems very strange how this problem only occurs once I hit 5000rpm.

kung fu jesus
01-20-2014, 12:43 PM
Is this a 1.8 swap? You say na8, which implies a 1.8. Is it as such or a 1.6? A '93 should be a 1.6. Just need that for clarification...

Does the hesitation/bog still happen after 4k?

The tach is driven off an electronic signal. This signal could be a loose/chaffed wire, or it is possible your ignition is breaking up at higher RPMs. Can't help unless I know if the hesitation is still there. Could be one of a few possibilites: bad coils, failing CAS, failing ignitor, rounded keyway - like the early 1.6s had issues with...yes it can happen to later engines and even 1.8s if the woodruff key wasn't installed correctly.

Also, the hesitation could be a clogged cat.

How many miles is on this car? Because is it a '93, is it a CA emissions vehicle?

cheyu
01-20-2014, 01:09 PM
Is this a 1.8 swap? You say na8, which implies a 1.8. Is it as such or a 1.6? A '93 should be a 1.6. Just need that for clarification...

Does the hesitation/bog still happen after 4k?

The tach is driven off an electronic signal. This signal could be a loose/chaffed wire, or it is possible your ignition is breaking up at higher RPMs. Can't help unless I know if the hesitation is still there. Could be one of a few possibilites: bad coils, failing CAS, failing ignitor, rounded keyway - like the early 1.6s had issues with...yes it can happen to later engines and even 1.8s if the woodruff key wasn't installed correctly.

Also, the hesitation could be a clogged cat.

How many miles is on this car? Because is it a '93, is it a CA emissions vehicle?


First of all thanks for the reply! Sorry for the confusion, my car is an original 1994 1.8 sold in Thailand (far far away).

Based on what I have noticed, it only occurs after 5000rpm, which is very strange to me. It would make more sense if it starts occurring from 4000 since that's when the engine switches to open-loop control.

I don't think the problem is with the tach, because the car would start jerking once I reach 5,000rpm and you can clearly hear the engine speed bouncing around (if you watched my video on youtube).

What do you suggest as a plan of action? Sorry, I am a fairly inexperienced mechanic. One of the reason I got this car was to use it as a training platform.

The car has nearly 140,000km to it.

kung fu jesus
01-20-2014, 01:26 PM
Watched the video again with the sound turned up. I am leaning towards the ignition system. Coils and/or ignitor are probably weak. Are you able to pull any codes?

cheyu
01-20-2014, 02:31 PM
Watched the video again with the sound turned up. I am leaning towards the ignition system. Coils and/or ignitor are probably weak. Are you able to pull any codes?

I'll let you know once I get to it. :)

WASABI
01-20-2014, 02:49 PM
You said you fitted new spark plugs... did you also get new wires as well? The NGK blue wires are good for 30,000 miles or so. Also, I am going to assume the timing was done correctly? Has the timing belt been done? Correct me if I'm wrong but, if the timing was advanced, and then a new timing belt was installed, say, one tooth off, couldn't that give the appearance of a car running correctly until higher RPMs? And loss of power?