Engine ( 5sfe ) stumbles under load but maintains rpm okay

[uuuuu]

Hey guys, did a few searches but didn't find the exact same symptoms as I'm having. Check engine light is off.

I have a 1993 auto 5s-fe with 200,000 miles. Starts up and idles fine, but there is a sporadic miss once in a while. The meat of the problem comes consistently when I press down the pedal hard from a stop light (while in drive). There is lots of stumbling and hesitation and it is easy to stall the car. Same thing happens if I am in park and try to rev very quickly from idle (or any speed under 3000 rpm). But, if I slowly raise the rpms from idle, I can get up to high rpms and there are no problems. Also, if I start revving from ~3000 rpm and rev quickly, it raises engine speed well and sounds great. If I start revving from idle and it stumbles and catches itself a few times, it will work great once it reaches past ~3000 rpm.

I checked the spark plugs, they seem to look fine and are gapped correctly. I have over 1/4 tank of gas, just filled up with 93 octane from Marathon to confirm that it isn't bad gas.

When it stumbles, it sounds the same as when you rev the engine high and the shut off the ignition (pistons just compressing air, no combustion).

My first thoughts lead toward fuel issues. I am in the process of getting some fittings to measure fuel pressure (it is a strange banjo bolt setup), so that should give me some answers there. Is there any evidence to suggest either fuel pump or fuel pressure regulator? I changed out the fuel filter (I don't think it had ever been done before!) and the problem persists.

My next thought might be some engine control system where the effect changes at a certain engine speed. For example, the EGR system. From the Haynes manual:

With the coolant temperature below 131-degrees F (cold), verify that the vacuum gauge indicates zero (no vacuum) at 2,500 rpm.
With the engine warm, verify that the vacuum gauge indicates low vacuum at 2,500 rpm.
Disconnect the vacuum hose from the R port of the EGR vacuum modulator and connect the R port directly to the intake manifold using an extension. Raise the rpm to 2,500 and observe the vacuum gauge indicate high vacuum.

Does anyone have any ideas? I'm also looking for a good, solid gauge that will measure both vacuum and positive pressure. It would also be nice to be able to apply both vacuum and positive pressure through the use of a hand pump. Any suggestions?

Hopefully, when all is said and done, this will help someone else with the same problem.

[JohnGD]

Fuel pressure, filter restriction?

As far as EGR goes you can give gas while a helper pull vacuum on the valve to open the EGR valve. If the problem lessens or goes away, then you know it's EGR.

[uuuuu]

Did a battery of tests today, signs are pointing towards spark problem. Short update: Had a buddy hook up a timing light to each spark plug wire and there is no spark in some cylinders when the engine starts to miss. It's not just one cylinder that does it, so the 'miss' is spread among the cylinders. I would think that means something is wrong at the distributor or before. The distributor is less than a year old; the spark plugs (these are Bosch...I've heard bad things about Bosch plugs in our cars...) and cap/rotor are about 2.5 years old. The spark plug wires are several years old and are very cheap-looking. I may start with that.

Other testing:

Hooked up a pressure gauge to the fuel line and saw consistent pressure in the mid-40 psi range, upwards of 47-48 during the periods where the engine is missing.

At idle, applied vacuum to the fuel pressure regulator and fuel pressure dropped some but not below 38ish.

Checked the MAP, was reading 3.5 volts with atmospheric pressure and 0 volts with 25" Hg (Haynes manual says approx 4 volts and 0.5 volts, so I think that is close enough).

Checked the intake air temperature, resistance was around 500 ohms at the time. Cooled down a bit and read 800 ohms.

Applied vacuum to the EGR valve at idle (which opens it and allows exhaust gases to enter the intake) and the engine stalled, so I think the EGR valve is working properly.

[Mike Gerber]

Most missfires are caused by faulty ignition system components. Since you have missfires on multiple cylinders and you said the spark plugs look OK, I would suspect the distributor items. Remove and inspect the distributor cap for signs of cracks and arcing. Next remove the rotor and inspect it for cracks, especially underneath where it sits on the distributoor shaft. Then check the coil for being in spec. You will need a digital volt/ohmeter and the specs from any manual to do this. Finally, and this is probably the most important thing, remove the coil from the distributor and inspect it carefully for cracks. This is a trouble area on the first 3 generations of Camry, as these coils rack up lot's of miles. I know your rebuilt distributor isn't that old, but I have seen poor quality parts used on aftermarket rebuilt distributors.

Mike

[uuuuu]

I think I found the problem! I rotated the distributor assembly clockwise (looking at it from the driver's side) and it doesn't miss anymore! There's a good chance I messed up the position of the distributor when I replaced it last year...although I'm wondering why the symptoms are only showing now...

Can anyone confirm that rotating the distributor assembly clockwise allows for more advanced ignition? My understanding is that under high throttle, the ECM tries to advance the spark (and my rotor wasn't close enough to a post in the distributor to actually fire when it wanted to). That way, idle speed and cruising speed would maintain a 'normal' (not advanced or retarded) spark and my engine would skip / stumble / hesitate / miss under high throttle.

[Mike Gerber]

"Can anyone confirm that rotating the distributor assembly clockwise allows for more advanced ignition?"

That is correct. I would try to get hold of a timing light and set the timing properly to spec. If you have Autozone in your area, try to see if they lend out a timing light within their tool lending program.

Mike

[JustinC25]

Sounds like a bad coil, I had this problem last year in mine.
Big crack right across the top. At low load it was fine, but under load the coil had to work harder, and it arc'd out of the crack. Remove the distributor cap, and take a look at the coil.
If it's bad, then change it and I'd do a cap too.

[uuuuu]

Looked at the timing @ idle after my "fix", it was way too advanced. There must be something else going on...

Decided to replace distributor cap and rotor (neither fixed the problem), also replaced spark plug wires (one wire hadn't shown any continuity...still didn't fix the problem) and then CHANGED MY BOSCH SPARK PLUGS TO NGK IRIDIUM AND IT WORKS AWESOME AT 12 degrees BTDC!

Keep in mind that these Bosch plugs were less than 2.5 years old and had less than 15,000 miles on them. Here's just one more anecdote to show that Bosch plugs in our cars don't work well.