1990 Nissan Sentra 1.6L 4speed - followup on stumble during acceleration

The problem is clearly being caused by something that depends on engine vacuum. When vacuum is low, the problem appears and then disappears when vacuum is high. I'd check the EGR system. Too much EGR will lean out the mixture on acceleration and cause stumbling and hesitation. You can test this hypothesis by blocking the vacuum line to the EGR valve thus disabling the EGR system. If the problem disappears, you know it's the EGR system. Another characteristic of EGR system trouble, is that the stumbling/hesitation will not happen when the engine is cold and warming up but rather begins abruptly at a very specific coolant temperature during warmup. That occurs because the vacuum for the EGR system is supplied through a themostatically controlled vacuum switching valve which only opens above a specific coolant temperature. Good luck. Al

Thanks for the advice. I will test this out using your theory and plug the egr valve and giving it a test drive.

You are testing at wrong place. You are testing between feed voltage (constant 5V) and output. Test is between output and ground.

Sounds like a fuel starvation problem. Really needs a fuel pressure test under load. T a pressure gauge in after filter, on a long hose. Route the hose under back of bonnet and place pressure gauge so it can be seen from drivers seat.

When I had this problem, I went though the whole fuel system. Everything pointed to a faulty connection but I couldn't find it. Then it packed up completely. Power at relay, no power at pump. Then I found the alarm, dry joint and quite well toasted. Some dork had put a

Darned Haynes manual! those pictures are never accurate! I will retest. Thanks.

robbie wrote in news: snipped-for-privacy@g28g2000yqh.googlegroups.com:

If you haven't checked already, inspect any air hoses leading to the throttle body carefully. If they are cracked and open up when your engine flexes the motor mounts as you accelerate, then it may be screwing up the air mixture.

Andrew Chaplin wrote in news:Xns9D26805F941AAbluegriffin@216.196.109.144:

Heh,they just talked about this on Motorweek yesterday...about the air duct from the MAF to the throttle body cracking and opening under motor torque.

I tried the egr valve bypass and it definitely helped, but there is still some slight hesititation. I am going to reinstall the fuel pressure gauge and mount it so i can test drive it while watching the gauge as someone else has suggested. I am convinced that the egr valve is part of the problems. Anyone know of any reason (except emissions) to get the egr system working properly versus leaving it bypassed? Thanks!

How about a blocked cat converter? My 87 Pulsar 1.6 had that problem. I'm not sure, but you may have 2 of them (the Pulsar did). One at the exhaust manifold and one in the usual location, below the drivers seat If only one, it's in the usual location

That is, in fact, where I got the idea, and it may fit with the problem he's having with his Sentra, especially since it is "stumbling during acceleration." It's an easy thing to check.

-- Andrew Chaplin SIT MIHI GLADIUS SICUT SANCTO MARTINO

In message , robbie writes

The EGR system helps purge the crankcase of fumes and any pressure build up caused by piston blow-by. Doing away with this could cause crankcase pressure and oil leaks.

No. Your thinking of the PCV valve, The EGR recirculates exhaust into the cylinders to lean out the mixture, warm the incoming air, and cool the burn so as not to burn valves

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UPDATE: Thanks for the multiple threads of advice- ALMOST too much to try to troubleshoot!

fuel pressure- I put a t-connector in after the fuel filter, and mounted it where i could watch while driving- it never got below about

- is that enough fuel for WOT or is that where my problem lies? Thanks!

Mode III faults that result in non 55 code tend to be gross faults like something unplugged or shorted. When the code isn't 55 the check engine light will be on.

Some Nissan ECU's have a mode V - dynamic test. Doesn't store any fault for mode V faults. The flash pattern indicates which system the fault is in.

Sorry I don't have this data for Sentra 1.6, only UK/Euro '89-94 CA18DET. Sentra may be different. CAS missing pulse - even spaced slow flashes on 1.6sec off 1.6sec. AFM momentary spike in output, 2 flashes then a gap, on 0.6sec, off

Have you measured manifold vacuum? It should be 18-20" Hg at idle and more importantly, it should be very stable. It should drop as throttle opening increases and should not be fluctuating wildly. If all of that is normal, I'd go back to the EGR system. You said that when you disabled it, things got better. If everything else checks OK, that's the only cause/effect relationship you have. A very lazy or a partially stuck open EGR valve, especially if there's a vacuum leak at the gasket between the EGR valve and the intake manifold, could be the problem. Good luck. Al

One other thing I have confirmed through multiple test drives- the symptoms do not occur at startup- they begin when normal operating temperature is reached. when cold the acceleration is strong and uninterrupted by any stumbling or hesitation whatsoever. so whatever is occurring is temperature dependent. i believe someone mentioned temperature sensor a while back on another thread- are there more than one temp sensors? my temp gauge works correctly by the way.

Yes there are 2 temp sensors. One single wire one for temp gauge and a