1999 Altima hesitation off idle, pls help!

[merc68k]

Hi guys,

I've got a 1999 Altima (100,000 kms) with a miss off idle. If I tap the gas at idle, the revs drop slightly and nothing happens for a split second before the engine responds. I've been trying to figure it out since I got the car almost 2 years ago and I've done the following so far:

I've checked for vacuum leaks and I'm relatively sure there are none.

I put a vacuum gauge on the fuel pressure regulator's vacuum line and it shows 22 inches of vacuum at idle. I did notice that the vacuum is not smooth at idle, it wavers back and forth quickly maybe +/- 1 inch, about 6-8 times a second. Once off idle, this smooths out, which seems a bit strange to me. The idle itself is smooth.

I've checked the fuel pressure, and it's within spec. The regulator behavesas described in the service manual when I disconnect the vacuum line.I tested the MAF sensor and its output voltage rises smoothly with anyincrease in throttle opening.

I tested the TPS and it operates smoothly and is within spec.The coolant temp sensor and intake air temp sensor both are about rightaccording to the service manual.

It has clean plugs, newish rotor, cap, and wires, and is not burning oil orcoolant.

The air filter is relatively clean and the fuel filter is new.

There are no trouble codes stored, and aside from this problem and having aslightly high idle, the car runs smoothly and has good power at all otherrevs... although there is some hesitation at times at very low revs which I assume is likely related to what's causing the idle hesitation.

The problem manifests itself as a drop in revs/stumble when I tap thethrottle off idle. It's most obvious with small abrupt throttle openings,the kind you would use to take off from a stop (it's a manual trans).It's more severe when the car has been started between 5 and 10 minutes ofhaving been shut of and is still warm. With a shorter or longer stop, itdoesn't tend to be any worse than normal.

I tested the O2 sensor as well as I could with the tools I have; my analogmeter's lowest range is 10V, so it's hard to accurately monitor the sensorwith it, and my digital meter I suspect is a bit slow to show the voltagechanges from the sensor. But, with the digtal meter, the voltage seemed tosettle around .2 volts at idle, and would spike up to around .7 if I revvedthe engine. I measured the resistance of pins 1 and 3 (the heater element Isuspect) of the sensor, as per the service manual's instruction, and itreads almost 8 ohms hot, and 4.6 ohms or so at around 5 degrees C.

The service manual says it should fall between 2.3 and 4.3 ohms at 25 deg.C, so I assume the idea is the resistance increases with temperature. Ifthat's the case, then at 25C, I would expect the resistance to be more than4.6 ohms as I recorded at 5 degrees, so that is somewhat suspect.

I tried the diagnostic mode built into the ECU where the CEL blinks on and off as the sensor cycles between rich and lean, and at 2000 RPM it happens about 10 times in 10 seconds, but at idle, it might happen 1-2 times in 10 seconds.

I priced a new sensor at around $100 CDN today from the dealer, so I'm halftempted to install one. If it doesn't fix the problem at least it will benew and might help my fuel economy a little although it isn't poor to beginwith.

Any thoughts/suggestions as to what else I might look at?

[nismo96ka]

ok well the 02 isn't going to switch very much at idle... the test says to hold the rpm at 2000 and make sure it switches at least 7 times in 10sec. in my experience you're problem is usually the MAF... 9 times out of 10.

[merc68k]

I've sort of suspected the MAF too, but the limited testing I've done on it hasn't shown anything odd. I've tested for a voltage differential between its ground and the chassis, there was none, and I measured the voltage on its signal pin back to the ECU (measured at the sensor) and it increases with any throttle opening.

About the only thing I'm still a little unsure of is the coolant temp sensor... but the fact that the cooling fans do switch on when the car warms up, and there are no codes set would suggest it's working fine.

I've tried advancing the timing by turning the distributor, and this decreases the miss when I goose the throttle, but it's still there.. maybe that provides a bit of a clue? Obviously increasing the timing would help response overall, so maybe that's a bit of a red herring.

[tinaslynn]

I had a similar problem on my stanza. At first, thought it was MAF. I had always had a bit of a wacko idle, but it never hesitated or ran bad. Suddenly it began to want to kill out on acceleration from a complete stop. Two things I found, not sure exactly which was it... replaced TPS (car idles like a champ now) and found a cracked vacuum hose on the bottom side of the air intake tube. Maf is fine.. never replaced it (but did buy a used one to be put in later) and car takes off and run great now. I did also replace the CTS but do not think that was the problem. I also cleaned out the throttle body (made for better power on take off too). I had no codes either... it was intermittent and could not be duplicated no matter how hard I tried, but it started to happen more often (every time I drove it and usually after it warmed up). I bought the TPS from carquest for about $24...Good luck!!!

[merc68k]

That's interesting. I've tested my TPS up and down (literally) and it seems fine, there don't seem to be any dead spots in it and everything seems to be adjusted within spec.

It does seem like that sort of a problem, but unfortunately the screws on myTPS are almost impossible to get to and and won't budge.. I've started to strip the head on one of them and stopped to avoid totally wrecking it. I think I could probably get it off if I took the throttle body off, but that's a big job just on a hunch.

My problem is pretty much always there, although it seems to vary in intensity. It's normally worst just after starting the engine, and seems to lessen after the car has been running a while, even if it was already hot when first started. Also, the idle is normally lower when it first starts, and will stay at that speed indefinitely until I drive away, which makes me wonder if the ECU is trying to compensate for the miss.

I spent some time this weekend going over everything I can and testing all the parts I could think of, and was able to verify the MAF is good, CTS is good, MAP is ok, and like I said, the TPS seems ok, so I'm not sure what's left.

I suppose it's possible that there is a slight deviance in the TPS's sweep that my analog and digital meters both didn't see, did you by any chance put a meter on your old TPS before you swapped in the new one?

Thanks for your post.

[Altimasurgery801]

Dude Im probably a freakin idiot compared to you folks but I had the same exact symptoms to a T. I took it to tunex and it took three visits to pin point the issue and it ended up being the mass air-flow sensor. so um yeah just some dumb guy finally able to give some advice.

[merc68k]

Dude Im probably a freakin idiot compared to you folks but I had the same exact symptoms to a T. I took it to tunex and it took three visits to pin point the issue and it ended up being the mass air-flow sensor. so um yeah just some dumb guy finally able to give some advice.

Hey thanks for the post -- do you know how they were able to figure out it was the maf sensor? Mine tests out ok according to the service manual. Did they just swap it out on guess and it fixed the problem?? Can you describe all the symptoms you were getting just for the sake of humoring me?? Thx

PS, what year is your Alt?

[tjackson]

Hi,I am having the same issue.Was this issue resolve and would you please let me know what was the fix?Thanks.

[merc68k]

Hi, no I never did find out what the problem was, if any. I drove a '98 Altima, also a 5 speed, and it had the same hesitation from idle, maybe even worse than mine, so it may just be inherent in the ECU programming of that era Alt.

I've since sold the car, but to my dad, so I may still find out what the problem was, if it ever gets any worse.

Does your car have any other odd symptoms like idling up to 1500 rpm for a few seconds when you come to a stop and press in the clutch (if manual trans) or revving up and down a few times when you start it?

[tjackson]

Hi,The other symptoms are I can only get the timing to 10 deg btc.If I disconnect the tps the car will start but only run for a second or two.According to the Haynes manual I need to disconnect the tps to set the timing. Also I had to replace the knock sensor, no help.Thanks.

[merc68k]

Hmm, mine would run with the TPS unplugged. Are you disconnecting both connectors?

If you give me your email address I can send you the factory service manual, that might help you find the problem.

[tjackson]

Thanks,[email protected].

[tjackson]

Hi,I only received part 1 of the e-mail.Also I have an automatic transmision, and I did disconnect both connectors on the tps.Thanks for all your help.