Stan944

The engine hesitates and "gently" backfires, especially above 2000 rpm [EDIT: on cold and also hot engine, i.e. anytime] on cold engine. Once it almost died at an intersection, but it kept rough idling at about 400-500 rpm.
Her's what I tried to isolate the problem:
1. The tachometer never goes down, i.e. it shows the actual rpm, so the flywheel sensor (the one that is important during normal operation) is OK.
2. I connected a voltmetr to the air flow meter - looks reasonable - no unexpected drops in voltage, etc.
3. I connected a voltmeter to the engine temp. sensor - the voltage is normal. A break in wireing for this sensor would cause the engine to die.
4. This morning I checked the vacuum, and it looked low at idle (~15 psi). The scale says this is the regime of late ignition timing. Any comments here?
5. The DME relay must be OK, as I have an LED connected out there, and it shines steadlily.

Any suggestions for further testing would be appreciated.
thanks, Stan

RMills944

sounds like an exhaust peoblem - a clogged cat maybe.

Chico Valdez

I would check timing. Check to see if your distributor has rotated, Maybe your timing belt slipped. There is a screw that holds the rotor in place on some models. If it comes loose things get weird.

944CS

i believe this happens when the engine switches from open loop to closed. If you have an air/fuel ratio gauge you will notice that the gauge will start toggling (working) just as the car hesitates like this. Does this happen on a warm engine too, after about 45 seconds from the time you turn it on?

Stan944

The rotor is tight. I'll check other ignition parts.

is there a way to force the system to be in one loop only. I think I could disconnect the oxygen sensor to test it. what do you think?

clogged exhaust? I think I'll check other items first.
RMills944: would you elaborate on your experience with such cases?

944CS

Yes by undoing the O2 sensor you force the car to be in open loop. This is bad for fuel economy and the car will also not run well. The way cars came from the factory without catalytic converters and O2 sensors is the way you want to model your setup. For permanant open loop you must buy a coding plug, remove the catalytic converter ( I would suggest adding a pre-muffler), and ground the O2 sensor wire.

You cannot force the car to be in closed loop all the time, the oxygen sensor needs time to heat up. Even if you had the car warmed up, shut it off, and turned it on right away the computer still waits 45 seconds until it switches to closed loop.
You can reduce the time it is in open loop (cold motor) by purchasing a 3 wire heated sensor like california cars.

bleucamaro

I'm with 944CS on this. Diagnose your O2 sensor. Also, check your FPR.

dgz924s

Check for vacuum leaks and a dead spot on the wiper in the AFM. Had the exact issue and this was the fix. In my case the J boot was the guilty party and the magnetic wiper strip was worn/flat spot.

Backfires are caused by a vacuum leak or a timing/electrical problem, as in plugs/wires if one is faulty fuel can load up in the cylinder and spark is off it will still ignite but in the form of pre/late detonation or backfiring.

The hesitation is caused by the AFM wiper material chips or grooves out too deep and causes the pick up to miss a spot, causing the hiccup.

If you have an ISV, disable it at the terminal plug near fender. Fire it up and spray brake clean around the vacuum lines/intake manifold to head and throttle body.

Dal

nickg

you may have an aged air filter,,,

Stan944

I would think a partially clogged air filter is not a problem unless fully accelerating, because the air flow meter is past the air filter. Am I wrong?

Stan944

I think I have both backfiring (which I can hear only when stopped) and hesitation (while driving). I would think backfiring would cause hesitation? Am I correct?

I don't think my AFM is bad because I monitor the voltage on it, and it doesn't show UNEXPECTED drops, e.g. I rev it at 2k rpm while standing, and the AFM voltage is stable.

I'll examine the rotor/distributor cup.

dgz924s

So you used terminals 7 and ground to measure voltage using the barn door? And/or terminals 22 and 6? The cap and all can cause a miss but that is different than a hesitation. If I am on track I would say your hesitation is a one time thing while on the gas in a very tight rpm range and when letting off the gas ever so much to upshift you hear a pop!?

Dal

Stan944

I just edited my first post in the thread: it happens independently of the engine temperature. The backfiring is most heard around 2-3krpm, but could be at higher rpm too - I just can't hear it. Not at idle though, except for one instance that it almost stalled.
I hooked up many test wires to the DME harness a long time ago. I don't remember which terminals. The air temperature in the AFM is steady too.

I don't have much experience with 'cranky' engines, so my terminology might be off. I call it backfiring, because I hear low-pitch explosions in the cabin, every second or so, although not regularily.
I call it engine hesitation, because the car's movement feels 'jerky' while driving. This is rather intermittent, but I feel it to some degree most of the time now. But this might not be 'hesitation' people refer to commonly.

Regarding open/closed loop operation: I get same symptoms when the oxygen sensor is disconnected, so I don't think it's toggling between the open and closed loop.

yellowline

At least look inside the AFM. FR Wilk also describes soldering a redundant wire inside to reduce voltage loss from the wiper arm to the output lead. I did this over the past weekend, and an AFM service cured 95% of my poor idle and missing problems. It can't hurt to clean contacts and examine the potentiometer.

944CS

do you have platinum +4 plugs?