Bad fuel economy on 94 Sentra

ANDREIR

MEMBER
1994 NISSAN SENTRA

Hi, I'm new on the forum and I hope to get some help here.

Here is my problem:

(I apologize if it is a little bit long, but I want to give all the details so that I can have the right kind of advice)

My 94 1.6 liter sentra has 240 000 km (150 000 miles) and about a month ago my fuel economy dropped from 7-8 liters/100 kilometers to 12 liters/100km (sorry I don't know what it is in mpg). And the check engine light came on not so long after that. I checked the trouble code and it was the EGR valve malfunction. So I got myself a new EGR valve (a nice 140$ out of my pocket) and the problem seemed to be solved for about a week and the frist gas tank after that. I did the normal 8L/100km. One week later the check engine light came on again and I started noticing that the economy dropped again. I checked the trouble code and was the EGR valve again, but then, the same night, the light went off and didn't come back. And now the ECU shows normal functioning. But since then, my fuel economy is around 10L/100kms which would be fine for 2.4 liter engine or something like that but not for my tiny 1.6, which I rimind did always under 8L/100km. Also, I am absolutely sure that I don't have any gas leak and all my fuel and air filters are fine (which doesnt change much by the way). Besides that, other sensors such as O2 sensor and the ECT give the correct readings. Today I did a compression test and it turned out to be 180 psi for all 4 cylinders which is great for the age and mileage of the car. One detail though, when the compression test is performed, you are supposed to keep the throttle fully open so that the air can get in the cylinder. I forgot about this detail, but still got a good reading. But I heard that if you don't open the throttle the reading would be way lower. Is it true? And doest it mean that my throttle body is leaking air (sucking more even when closed) or it's something totally stupid and not to worry about? So what could be the problem that changed so suddenly and so radically my car's fuel economy? Can it be the new EGR valve that doesn't work as it is supposed to but doesn't set off a check engine code? I could go try to exchange it, but I want to be sure before I do that.

If someone can give me a precious advice, I would greatly appreciate. Thank you for your attention.

7 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Poor Gas Mileage

When is the last time this Nismo seen a major tune-up? It could be a lot of things causing it to eat gas like water.

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Sunday, February 11th, 2007 AT 3:43 AM

ANDREIR

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7 POSTS

I never really do a "major tune-up" once at a time. I replace things gradually and always make sure evrything is okay on my car. My spark plugs aren't that old: maybe a year or little more (i have platinum NGKs). I check them from time to time to see the condition and until now they are perfect and I also make sure the gap is right. The wires are about almost 3 years old, but still in good shape. I have no idea about the rotor and the distributer cap though. Will check it out today. Could this be a possible cause, if the spark isnt good? But then the spark plugs would be dirty, no? The air filter is good (i have a short ram air intake, and don't start telling me that it eats more gas because of that, I have it since last may and it never affected the gas mileage). The fuel filter was changed two weeks ago.

Sometimes I hear that this problem could be due to a bad timing. If the timing chain skips a notch. I don't really think it's possible though. Is there way to check if it's the case?

Also the catalytic converter might be blocked. The catalytic converter and the manifold are the only two parts of the exhaust that I haven't changed yet.

But if it's one of the two last causes, I would probably notice that the egine works differently.

But I am still mainly suspecting the EGR valve because as soon as I replaced it, the problem went away for about a week. It could also be the carbon canister control solenoid, but I measured it's resistance as it said in the repair manual and it gave me a satisfying value.

So i'm still lost and confused.

Thanks again.

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Sunday, February 11th, 2007 AT 5:48 PM

RASMATAZ

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Here's a partial lists

Coolant Temeprature sensor locked in Open loop

Retarded Ignition Timing

Clogged C/Converter

Excessive Fuel Pressure

Weak Ignition coil

Open injectors

Overheating

Suspension and Brake problem-causing resistance

There's more to come as you wish-its about 5 pages long

The EGR if its opening too soon-the O2 sensor will see a lean mix and the ECU will dump more fuel.

Any OBD1 Code-check for codes don't wait for the idiot to turn On.

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Sunday, February 11th, 2007 AT 7:52 PM

ANDREIR

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As I said in my first message, the ECT sensor works fne. In the case of my car it doesnt have an open or closed loopp as it sends no voltage. It is a thermistor which chages resistance according to the temperature. I measured it with an ohmmeter at different temperatures and the readings are withing specifications.

The O2 sensor can stay however locked in open loop, but it's not the case either. I verified that and I am pretty sure of the results.

There is no overheating (at least the temperature guage says so).

About the mechanical drag. Hmm I know that my e-brake gets stuck sometimes (it works only on one side and jams, but I never use it), so i'll check that just in case

So what's left from what you said for now is:

Retarded Ignition Timing

Clogged C/Converter

Excessive Fuel Pressure

Weak Ignition coil

Open injectors

All of these I am not sure how to find out if that's the problem. And it starts to smell expensive.

Do I have a fuel pressure regulator on my car? If no, the what? The pump itself gives too much pressure suddenly?

About the OBD1 check, isnt it the equivalent of a self diagnosis? That's what I was performing until now. Or there's more trouble codes that could be seen with an OBD1 check? And I can do it without the check engine light beeing on? Because for now with the light off, it shows code 55 which is the normal operation of the MPFI system. Before that was code 32 for the faulty EGR valve operation, but I reset it after the light went off by itself, to see if it will persist and turn the check engine light on again (until I reset the ECU codes the code 32 was still there even with the light off, but I think it doesnt mean that the problem is still there, it just keeps it in memory, correct me if i'm wrong). This is why I am not sure whether I should try to get the EGR valve exchanged (i am not even sure they will want at the auto parts store).

I really appreciate those advices, the thing is that I am a student and I cannot afford changing everything just to find out that the problem is still there.

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Sunday, February 11th, 2007 AT 9:06 PM

RASMATAZ

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Tracking your problem could be a bear and worst than an nightmare like the needle.

Lets try the EGR again-get in there look for carbon and all kind of stuffs, make sure it gets vacuum and solenoids are operating properly. If you cleared the RAM and its still there-something is totally wrong. Maybe the EGR/Wiring and the ECU problem.

Some more on the lists

Faulty Ignition components

Loss of engine compression

Defective Valves

Excessive rolling resistance from low tires, dragging brakes, wheel misalignment etc.

Clutch slipping

Transmission slipping or not upshifting

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Sunday, February 11th, 2007 AT 9:39 PM

ANDREIR

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7 POSTS

Believe me, I already have nightmares about this problem. But actually today, I did about 120 km for the last quarter tank which isn't so bad. It is around 9l/100km.

My tires are all well inflated and I have no rolling resistance. The clutch slips though, but only if I floor it. In normal driving there is no slipping.

The trouble code isn't there anymore after I reset it. So there's no problem with the ECU.

I measured my compression a couple days ago and it's 180 psi for all four cylinders. Which I didn't even expect to be so good.

The defective valves would create a loss of compression, wouldn't they?

About the ignition components, I still haven't checked the cap and rotor. And I don't know how to test the ignition coil. If it's not strong enough. Do you have some test to suggest?

Also you mentioned once the open injectors. Is there a way to verify that also?

What I am suspecting now is that when I got the new EGR valve, there is a piece of meta pipe that screws in it and the a vacuum hose connects on it. Originally when the pipe was screwed in to the max it stopped in a horizontal position facing the vacuum hose. But on the new valve, the threads are cut in such way that when it's screwed in completely the pipe stays facing up and the vacuum hose doesn't reach. So I had to unscrew it 3/4 of a turn to put it in a horizontal position. From there might be a loss of vacuum. It is a small detail but could it be causing the problem. I twill put some Teflon tape over the threads and come back to you with the results.

Thank you for your time again.

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Monday, February 12th, 2007 AT 10:13 PM

RASMATAZ

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Testing Coil:

Primary resistance

Get a an ohmmeter set at RX1 put red lead to pos. On coil and black lead to neg. Of coil, should read somewhere between 1.04-1.27 ohms.

Secondary resistance

Put red lead to neg. Of coil and black lead to center of coil should read 7,300-11,000ohms

If the reading is way too off replace it. Give a little tolerances.

Note: disconnect a plug wire and crank it over if its yellowish-its weak-you're looking for a bright blue snapping spark.

Injectors

You have to disconnect the fuel rail to check the flow-it should be cone shape when it opens-not breaking. Also when you on the key it shouldn't constantly be dripping-meaning its open.

Start the sentra and listen very carefully for the sound on each injector-if there's something wrong with it, it will make a different sound from the rest.

HTH Good Luck!

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Tuesday, February 13th, 2007 AT 3:35 AM