Diagnose intermittent hesitation?

1987 Toyota MR2 NA 1600cc 4AGE engine, mostly stock.

I've owned it exactly a year, and a few month's ago I started noticing hesitation on the highway. Driving along 65mph or so, steady speed, it would start hesitating and bucking a little. No check engine light, nothing weird on any gauges. If I hit the gas, it was fine. Let off a bit and it would do it again.

At first it happened after 30min-1hr of driving. Lately it's started doing it around town, like 35-45mph on regular streets. Again, hit the gas and it pulls like it should. Cruise and it feels like I'm letting off the gas and hitting it again repeatedly. The worst part, it's intermittent! My garage has never had it happen after an hour of test driving. At one point it also had an intermittent (I'm beginning to hate that word) problem with not starting, or cutting off while running. 4-5 times at work it wouldn't start, towed to garage and started on first try.

To diagnose THAT issue we ended up replacing most of the electric. New plugs, wires, distributor, module and coil. The starting problem disappeared and I hoped the hesitation as well. Nope, came back on the highway going to a rallycross 1.5hrs away.

The hesitation problem has never left me stranded, never died completely, always takes off when I give it the gas, and if I keep going the hesitation goes away after 5 minutes or so. Coming home from the rallycross it did it 3-4-5 times over the 1.5hr drive.

My non-mechanical experience says fuel filter? Possibly fuel pump? Since we systematically replaced so much electrical I'm leaning toward fuel. (Possibly air. MAF?)

I'd really like to stop this so I feel comfy driving 2hrs to more events. Any guesses? (The MR2 forums are fairly dead and fuel filter was the only reply I got.)

Thanks!

Could be a clogged filter, could also be a clogged injector or bad O2 sensor. Maybe a bad/dirty MAF. Fuel filters are dirt cheap, so is MAF cleaner spray. Sending your injectors off to be rebuilt is also pretty reasonable; I'd just go ahead and do it all for peace of mind.

I had a really similar issue on my Saab that was solved by rebuilt injectors, FWIW.

Fuel pump can be checked with a lab scope by current ramping it.. This will tell you the condition its in and if it needs to be done or not... You don't need it to be acting up to see if its bad or not...

MAF sensor, I am guessing on that age of a toyota is going to be a vain air flow meter style.. Not a hot wire... That can also be tested with a scope to see if there are any flat spots causing a problem... There isn't really any cleaning to do on a vain air flow meter... IF you post a picture of it I can tell you for sure what style it is..

Plugged filters/dirty injectors are just that and don't often cause intermittent problems like what you are experiencing.... If they are bad, they are bad all the time.. A shorting/open circuiting injector can cause problems, but usually they cause slight missfire/ruff running at idle.. This can be tested by current ramping though with a lap scope... You don't need the car acting up to see a problem with them...

O2 sensor should be looked at...If its miss reporting then it could be causing the car to lean out and missfire/hesitate.. I have seen O2 sensors that fail in a manner causing problems like you are describing..

I would double check ignition timing.. make sure its set properly...

I would also scope your tps to check for flat spots... If the tps has a problem you'll be able to see it with a scope with out the car acting up... If its a 4 wire tps, check its adjustment...

Fuel problem should get worse as fuel demand increases. So if you hit the gas and it is fine, I would doubt it. That said, doing the filter never hurts. Pump on that car is real fun, plan on a weekend to do it the first time. I would be looking in other directions. At a steady cruise if it is dying you are running lean, maybe a vacuum leak. Need more data. What would happen if you drove it with the O2 sensor unhooked? Also a vacuum guage in the car so you can see what is happening might help. Another weird but effective thing is a timing light wired up into the car. When the miss happens, hit the trigger. If you don't get a strobe the ignition is the issue.

I was thinking it was odd that it only did it sometimes, and hitting the gas it pulls like it should, and also thought that a plugged filter would be all the time, but I'm not very car-fix-it inclined.

I'm liking the O2 sensor possibility. And a few other things mentioned so far, thanks! (I knew I'd get real feedback here...)

It only happens after the car has warmed up, does that lean towards any particular problem area?

Jerry wrote: It only happens after the car has warmed up, does that lean towards any particular problem area?

CrazyCanadian covered most of it. Only when warm points to the 02 theory since it's out of the loop when cold. Try running around with the 02 sensor disconnected for a while. If the symptoms disappear, then it suggests an oxygen sensor. I have encountered this more than once.

So I figured out where the O2 sensor was (I'm learning) & disconnected. Roads are pretty crappy so I just started and let warm up/run for 5-10 minutes. No ck engine light, everything ran fine.

If this ends up being the culprit, what are the ramifications of just leaving it unplugged?

Jerry wrote: So I figured out where the O2 sensor was (I'm learning) & disconnected. Roads are pretty crappy so I just started and let warm up/run for 5-10 minutes. No ck engine light, everything ran fine. If this ends up being the culprit, what are the ramifications of just leaving it unplugged?

It'll run in a "safe mode" and will dump fuel and probably pull timing. I wouldn't recommend it. New o2 sensor for that thing should be $15 at most.

The other thing to check for is that Toyotas of that vintage through the mid 90s were pretty notorious for flaky IAT sensors, which can cause similar issues.

But, if you find that o2 sensor fixes it, then i wouldn't continue looking for issues where none exist.

92CelicaHalfTrac wrote:

Jerry wrote: So I figured out where the O2 sensor was (I'm learning) & disconnected. Roads are pretty crappy so I just started and let warm up/run for 5-10 minutes. No ck engine light, everything ran fine. If this ends up being the culprit, what are the ramifications of just leaving it unplugged?

It'll run in a "safe mode" and will dump fuel and probably pull timing. I wouldn't recommend it. New o2 sensor for that thing should be $15 at most. The other thing to check for is that Toyotas of that vintage through the mid 90s were pretty notorious for flaky IAT sensors, which can cause similar issues. But, if you find that o2 sensor fixes it, then i wouldn't continue looking for issues where none exist.

It will actually just run in open loop mode... There is no real safe mode... It wont affect timing, but your fuel millage will go down...

Finally drove it for about 45 minutes today. Until the negative battery terminal decided to not .... uh be connected to the terminal, and it died very quickly and suddenly. In the snow. On a hill. But not barefoot.

That was resolved, but the 45 minutes before that happened, no hesitation. Thinking that's the issue.

Good possibility. How is the EGR valve ? vacuum leaks are always a possibility also.

Well I finally drove it a bit this weekend, to put it in a car show as part of an SCCA booth. Both on the way there and back it started hesitating again, fairly bad and quickly, on the way home especially.

The O2 was still unplugged (had a manual fuel pressure regulator put in to get A/F ratio in line and new starter, wasn't sure the garage replugged it, but no still unplugged). Anyone know if there is more than one O2 sensor?? My Scion xB has a few I know. (1987 MR2 4AGE N/A)

Otherwise what next? Someone at the show mentioned TPS with a bad spot? Really annoying being so intermittent.

Someone else on an MR2 forum said it could be the MAF sensor. Which one is cheaper and/or more likely?

In reply to Jerry:

Jerry- the car is a standard '87, right? I really doubt that there's a real check engine light- those didn't really come into ECU's until very late in the 80's, but more the early 90's, and didn't become really good until '96.

As for the second O2, since' theren's no OBD requirement for the car, there would not be a second O2 sensor- so don't bother looking for it.

Now- for the possible help- what kind of hesitation are you talking about? Like it's shutting the fuel off every once in a while, or a missfire? My Miata had the former, so to me, it felt like decel fuel shut off- I cleaned the throttle position sensor and it went away. Good thing- the TPS on my car wasn't cheap. Since your original post describes the symptom of basically on/off throttle, this does seem pretty reasonable.

Look for that, or also look for intake leaks- especially if the air intake gets compressed when you accelerate. And the leaks will be downstream of the MAF. I don't think it's that, but something to look for.

The MAF and O2 sensor issue will feel different. Oh, and the fuel wont shut off for a closed throttle unless the engine is warm- so it's like the O2 sensor in that respect.

I like that IAT suggestion, too. I know that when the IAT on the Neon comes unplugged (or tears off its ground...) the car starts running like crap. I'm not sure what they would have done in '87, though, but OBDII and MegaSquirt will both enrich the mixture if thinks it's really cold out.

It's amazing how much of a crutch the OBDII scanner and TunerStudioMS has become, though -- I keep thinking that this stuff would be really easy to test by just plugging in a laptop or a scanner!

I wish I had a better description of the hesitation, to distinguish between fuel shut-off and misfire.

I'll be cruising along 55-65MPH, and it just starts hesitating and bucking a bit. like hitting the throttle then brake quickly. I wish I had fired up the GoPro on the way home. It'll do that for a minute or two then stop. 5-10 minutes later, does it again. Anytime it does it if I hit the gas it takes off normally it feels.

I'm going to plug the O2 sensor back in, since that isn't the problem. Need to do some reading now to find and check the MAF. We added a K&N intake & I wish I remembered if this started before or after that addition. The filter is REALLY close to the sidewall.

Also, because I'm not as learned as some, what is IAT?

In reply to Jerry:

Probably Intake Air Temperature. Can also be Manifold Air Temp. Or we also call it ACT.

The throttle sensor is easy to check- and since it's a contact sensor, it's very possible that there's a dirty spot where you use it the most.

Sounds like an IAT sensor to me

Looks like I need to Google TPS, MAF, and IAF. A-OK!

Bad grounds or a bad neg post connector.

I had an issue where my old 240sx had vtec, no bottom end but at a mid-high rpm point it would pull like crazy, very uncharacteristic of the motor. Found that you can wiggle the negative connector on the battery with ease.

Rx7s are notorious for hesitation in the 2nd gen for when the secondary injectors fire. 99% of the time its bad grounds.

Talked to my garage person, he's pretty sure it sounds like TPS. Said he's replaced many in Toyota's over the years. Checking my favorite MR2 person locally for a used one to test, otherwise I'll pop the $ for a new one I guess.

TPS is a good place to start. Fuel pressure isn't the problem or it wouldn't be going away when you ask for more gas by hitting the throttle.

I'll throw my wild-@ss guess in the mix. Check the timing chain for slop. The distributor is driven off the cam. During acceleration, the crankshaft is pulling harder on the chain than it is during steady-state driving. During cruise it could be letting the cam dance around, then it goes away when you accelerate because it pulls harder on the chain.

Crap. New TPS last week, test drove today and 20 minutes in it started hesitating on the highway cruising at 65mph. berkeley.

Back to the drawing board...