less power when engine warms up

[sethulrich]

My car runs great, pulls hard in the first 15 minutes of driving, but when the engine gets warm it is down on power. Idle is almost always fine, whether the car is hot or cold. Very occaisonally it dips, but that's usually in connection with starting up another accesory, and I assume that is because of increased load on the alternator. Also, it backfires. I'm not sure if that is on the intake side or the exhaust side. Backfiring usually happens if I free-rev the engine, such as on a downshift to match revs. Or if I rev it quickly from idle to 3000-3500 rpm, it may backfire. Also, I do not know if these two problems are connected. So, I have a few specific questions.

1) Is airflow in a catalytic converter affected by temperature? That is, could it flow ok when cold, and then flow poorly when hot?

2) What all sensors affect fuel mixture, and which of these sensors are affected by temperature? The ones I am aware of are O2 sensor, coolant temp sensor, and the MAF. I have already replaced the coolant temp sensor.

3) I don't think it is a dirty MAF, because that would cause it to idle bad and be sluggish regardless of temperature. Is this a correct assumption?

4) Does a bad O2 sensor ALWAYS make the check engine light come on? or is it possible that I have a bad O2 sensor even though the check engine light does not come on? (I know the light works, because it is on when the key is 'on' and the engine not running)

5) Could it possibly be an ignition problem? I'm sure my plugs and plug wires are ok, I have replaced them not too long ago. I have looked at my distributor cap, and it looks fine.

6) Could it be timing?

So, my thoughts are it COULD be the O2 sensor, if it could go bad without tripping the engine light. Or, it could be a bad MAF sensor, which I can check soon. Or it could be something in the ignition...which I wouldn't know how to check. I tried to ask very specific question, in hopes of getting very specific answers, not speculation or nonsense like "My uncle's girlfriend's brother had that same problem and he said..."

Thanks for your help!

[sethulrich]

I didn't even tell you...my car is a 1993 240SX. No engine swap, just the good ol' KA24DE. Only mods are Cold air intake, electric fan instead of clutch fan, HKS grounding wires. I have also removed the AIV, plugged everything that I unhooked from it. The problem has been there before all the mods were done, and none of the mods have made a difference.

[NISTECH]

Quote »1) Is airflow in a catalytic converter affected by temperature? That is, could it flow ok when cold, and then flow poorly when hot? [/quote]

Yes if the exhuast has a restriction as it warms up it can cause a signifacant loss of power but the cat specifically is not the only part of the exhuast that could cause it could be muffler too.

Quote »2) What all sensors affect fuel mixture, and which of these sensors are affected by temperature? The ones I am aware of are O2 sensor, coolant temp sensor, and the MAF. I have already replaced the coolant temp sensor.[/quote]

You have pretty much nailed the primary fuel control sensors.

Quote »3) I don't think it is a dirty MAF, because that would cause it to idle bad and be sluggish regardless of temperature. Is this a correct assumption?[/quote]

not nessesarliy the maf can have a decent idle but terrible accel. Heres a good way to see that senario in action. Disconnect the maf completely and start it. It should idle fine but once you hit the gas it pops and sputters and will almost not accel at all. problem though with this being your problem is, It wouldn't matter weather it was hot or cold when the lack of power is figured in. It controls fuel from start up till shut down [cold or hot].

Quote »4) Does a bad O2 sensor ALWAYS make the check engine light come on? or is it possible that I have a bad O2 sensor even though the check engine light does not come on? (I know the light works, because it is on when the key is 'on' and the engine not running)[/quote]

Yes it is possible since your car is OBDI it does not moniter O2 operation only makes sure it goes above a certian voltage and below a certain voltage. not that it responds quickly or that it cycles properly. it can be in failure state and still not throw a CEL.

Quote »5) Could it possibly be an ignition problem? I'm sure my plugs and plug wires are ok, I have replaced them not too long ago. I have looked at my distributor cap, and it looks fine.[/quote]

Yes it could if your plugs are weak or in the wrong temp range it could cause drivability hot. Their spark could become weak. If it were cap it would miss when it failed. In my sig is a link to find the proper NGK plug for your car if you dont have those I recommend you get them. plugs like bosch and autolite dont do well in nissans.

Quote »6) Could it be timing?[/quote]

Yes when the computer goes into closed loop timing control comes in. it could be dropping it back. Check your base timing to start off. make sure its in base mode when you do it. You disconnect the tps to get it there]

How bad of lack of power are you talking. Flat on its face or just a little sluggish? have you put a fuel filter on it yet? Alot of guys are putting on 300z filters.

[sethulrich]

Wow, thanks NISTECH for the good answers!

About exhaust. I think I might have a leak somewhere in the exhaust, but not a bad one. What effect could that have?

I already have NGK plugs in my car, but they are the kind with the V-shaped notch in the electrode (not the same as splitfires). I don't know if that's good or bad.

For the O2 sensor, is there some way I can check it? or should I just buy the $20 part and see if it does the trick?

I'll look into the timing issue.

It's not totally flat on it's face, it's just sluggish. maybe 10-15 hp down? Enough to definitely be noticeable, but not like coughing and sputtering and barely making it. It still runs smoothly, it's just noticeably weaker. Thanks again for the great info.

[240sxfan]

I have exactly the same problem. My check engine light came ON on my 1995 Nissan 240sx, I have 65,000 miles on the car, I did a diagnostic on the ECU and it gave me code 55, which means everything is ok. The car runs great after it warms up a bit, but once I get the engine hot(not overheated) it seems to loose horse power, the engine feels sluggish, and my exhaust sounds crappy, like a Honda, it makes an annoying mid-rangy sound. I don't know what it is, i'm about to install new O2 sensors and headers on it, i hope it's the o2 sensors, my car has OBDII. I wonder what it could be? it's annoying...

[sethulrich]

I should also mention that I have the 300ZX filter, but I installed it more than 10,000 miles ago.

I did the ECU check today. (man, it wasn't fun crawling under the dash and unscrewing the ECU to see that stupid LED and the mode selector!) In mode II, engine off, I got the following flashes: 1 1 1 4 1 3. However, I didn't see what the starting number was. It just kept repeating the pattern. I think that it is codes 11, 14, and 13, not 41, 31, and 11. (I don't think there is any code 41, that's why I think it is the former). Please let me know if I read the codes wrong!

11=crank angle sensor circuit13=engine temperature sensor circuit14=Vehicle speed sensor circuit

I just replaced my coolant temp sensor recently. Is there another temp sensor anywhere? Or did it just show the code because it hadn't been reset yet since I replaced the coolant temp sensor?

Edit: I checked the codes again and only got 11 and 14.

I really don't know what to do about the codes, now that I've read them, esp the vehicle speed sensor circuit??? my speedo works fine...it's about 5 mph off, but it works fine. I really don't have any idea. And crank angle sensor???? Help!

Thanks for answering my questions.

[sethulrich]

One other thing. I got the method for reading codes off of 240SX.org, and I also got what the codes mean off of there. I don't know how accurate that information is. Sorry to go on and on, I just want to give you as much information as possible so you can give me the best answer possible. Thanks again!

[NISTECH]

the source for your info is good. you may be right about the coolant temp. the speedo could be intermitten or not sending the signal to the ecm from the speedometer(not all that uncommon) Crank angle sensor is inside the distributor. do you have a check engine light in your car?? if the crank angle sensor were failing it would turn that light on and the car would run like total crap.

[sethulrich]

Thanks again, NISTECH! :-)

The check engine light is not on when the car is running. And it doesn't run like total crap, either. It's just wussy when it gets warmed up.

If what you said about the speedo is true, that wouldn't have the affect on performance though, would it?

I still have yet to check the timing, and I haven't done anything about the exhaust yet.

[NISTECH]

yes lack of a speed signal to the ecm can effect performance.

[sethulrich]

Where is the vehicle speed sensor? in the transmission? (I have a 5 speed).

[NISTECH]

it is in the speedometer head.

[sethulrich]

Ok...sounds like the speed sensor is gonna be fun to get to! :-P

About the crank angle sensor. You say if it is failing the car would run like total crap. Well...it doesn't run like total crap...but then why would it throw a code for the crank angle sensor? I was looking at Autozone, and they couldn't find the part for me. Napa couldn't either. Apparently not all KA24DE engines had the same crank angle sensor, because when he couldn't find it for my car, I told him to check for an Altima. They had it for the Altima, and also for a newer 240SX, but it was obviously a different part than for my car. So I'll have to do some checking online.

[sethulrich]

Help! Does a crank angle sensor go by some other name? Every website I look at does NOT have it for my car, in spite of claiming to be a "complete source" for nissan parts....I've looked for crankshaft position sensor, crankshaft angle sensor...I've even broadened the search to just "sensor"...and that gives me 02 sensor, Knock sensor, and some other unrelated crap.

[sethulrich]

And now I just discovered something else....the codes I found on 240SX.org are for the 95-98 240...and mine is a 93. So now I don't even know if the codes are the same!

[NISTECH]

the codes are the same for your car under the 2 digit codes. your crank sensor is part of the distributor. the speed sensor is part of the speedo head it would have to be replaced as a unit. just the speedometer not the tach and other gauges.

[sethulrich]

When you said "speedometer head"....I assumed you meant this was something in the dashboard...in the gauge cluster. Forgive my ignorance. So I've got the speed sensor, and I took the dashboard apart to get the speed sensor out, only to find that none of the plugs in the back of the gauge cluster are anything like the speed sensor I got.

The part I have has a cast aluminum case with a shaft sticking out of it, and a rubber o-ring...looks to me like it should go in the transmission somewhere, and the o-ring is to keep the transmission fluid from leaking. From that small aluminum part, there is about three feet of wire (2 green wires) and a plastic connector on the other end.

Can you please explain in a little more depth where the speed sensor goes? Or is it possible that I was given an incorrect part? Not sure how soon you can answer the question, but I'd like to get my car back together this weekend.

Thanks for the help.

Edit: I bought the part at Advance Auto Parts for about $70... if that means anything.

[NISTECH]

They gave you the signal generator for the speedo. That paice goes in the tail of the trans. not in your dash. What you are looking for is the speedo head itself{ the thing in your dash with the numbers and the needle that tells you how fast your going} that is the speed sensor not the generator they sold you. The part you got is commonly reffered to as the speed sensor in the aftermarket parts houses.

[sethulrich]

ok....thanks for that info. I suppose the speedo head is a dealer only part? ....either that or a junkyard part...(which would have no guarantee on it...)

[sethulrich]

One other quick question...

The code on my ECM is 3 slow flashes and then 4 quick flashes. I took that as 11 and 14....but is it possible that it is actually code 34? knock sensor? Does that fit with the description of the problem? The reason I think it might be 34 is that when I reset the ECM previously, I got 5 slow flashes, then 5 quick flashes (which I took as code 55, everything ok.) So, I just want to be sure I'm not barking up the wrong tree with my code reading. Thanks again, NISTECH for all the help! I could add my thanks to the "thank NISTECH" thread!

[NISTECH]

Yes it is a code 34, not 11 and 14. And that would be the knock sensor [pretty common on nissans for that code]

[sethulrich]

ok, thanks! I returned the part for a full refund. :-)

The knock sensor is on the side of the block underneath the intake manifold, correct? all I can see is a wiring harness sticking out near the oil filter, and the wiring harness is all covered with oil, I would guess from when I get the oil changed. So, if the wiring harness that connects to the knock sensor got a bunch of oil in it, would that trip the code 34?

[NISTECH]

yes it is on the side of the block under the intake manifold.

typically it is the sensor and/or its connection to the subharness that fails. If oil were to destroy the harness yeah that could damage it.