91 5sfe AUTO Hesitation after warmup

[Scorpo90]

1991 MR2 w/5sfe and AUTO Trans.

Car hesitates between 1K and 1.5K rpm after it reaches normal operating temp. It feels like either its short on fuel or has too much (I don't think is running rich since I checked the sparkplugs and they do not show signs of running rich). Lack of power within the above mentioned rpm band. Idles ok, and accelerates with great power past the hesitation zone. No foul odor when idling or smoke.

This is what I know is not working to spec:
Engine Coolant Temperature Sensor, changed today. Still have the same hesitation symptom after repair (It really got in my nerves!).
Fuel Pressure VSV, omh are around 40 (out of spec), ground is ok, but no switching when electricity applied to terminals (out of spec). have not been able to find the part. If you happen to know about this, please let me know if this can cause the hesitation and why?
EGR VSV, omh are within specs, ground is ok, no switching when electricity applied to terminals (out of specs). found the part but way to expensive, I'm wondering if the same part but for other years will work, the 92-on are less than half the price of the 1991. The only visual difference is the holding bracket. Anyone knowledgeable on this please help me out.

Rechecked the timing three times, wiring, ignitor, distributor, spark plugs, and all seem to be on spec.

Have not checked the MAP sensor or the Throttle Positioning Sensor yet, that's tomorrow's task.

Here is the symptoms before the current issue, the car alternator died on me while driving, my voltage detector indicated low voltage and the battery/charging light came on. this happened about 1 mile from my house so I drove back home. Disconnected battery, and cleaned the engine, covering electrical components. Did not got any water near the Throttle body area at all. Replaced the alternator.

The check engine light has not come on at all.

I really love this car, have owned it for 5 years and have replaced/enhanced almost every component on it except the engine and transmission, I've spent so much $ on it is not even funny!, it is a project car for a future engine and trann. swap. Right now I don't have the $ to complete the swap and still want to have the car drivable. If you have experience on the hesitation issue please help me out, I do most of my mechanic work myself and really don't like to take the car to the shop, unless when absolutely necessary (like when replacing the AC, cause I don't have the equipment to do it myself). Thank you.

[Scorpo90]

Replaced the EGR VSV same hesitation problem. Also, cleaned the throtlebody and check each EGR line for clogs, this is getting the best of me. Also, have checked almost every other forum you can think off out there... to no avail. Thanks.

[Scorpo90]

ok, next in line, ignitor and distributor amp and voltage tests, I'll rebuild the distributor regardless, but I'll post if i find anything helpful. thanks.

[ricky91mr2]

I don't have the hesitation you have but I have a sort of hesitation between 3500 and 4000 and it appeared after rebuilding my dizzy, my it is not adjusted correctly, try playing with it since it is easily acessable and play with it while running. I am not even going to bother with mine since I am swapping out the motor this weekend but who knows maybe this will help.. Sorry it took me so long to see this I have not been on here since I have not replied to any of these post latley and my email has not been notified of changes on the board. (besides the parts for sale list/forum)

[Scorpo90]

Hi Rick, Ifound that the 5sfe ignition has a condenser, right by the ignition coil, so I took it out, could not get a resistance reading on it, is like $20 so I'm replacing it, I'm putting in a new coil since mine game me a ohm reading of 0.7 instead of the factory 0.4-0.5 range on the primary, the secondary was within specs. Since I'm at it, I'll rebuilt the distributor and get new plugs and wires. Since all my problems started after a failed alternator,I decided to take a second good look at all electrical components and finding this little discrepancies when compared to factory specs. By the way, I want to do a transmission swap, if you are closeby, are you getting rid of a 5sfe manual? thanks.

[ricky91mr2]

yeah I will be getting rid of the 5s manual tranny I have when I finished the swap, but I have already promised it to another mr2 owner here in Jax who does body work in exchange for fixing my messed up quarter panel.

[Scorpo90]

Ok after new distributor $300, new coil $80, new wires and spark plugs $120, still no new condenser, has not arrived. The hesitation is a lot less than before, still there a little bit, but not as bad. My recommendation to anyone that has a 1991 5sfe, is to dump that good for nothing piece of Jap cr@p engine, and do the swap to a 3sgte. Why, because the 3sgte has more parts available, since it was more popular, and definitely is a better engine. I don't have the $ to do the swap, so either I'll keep my car in the garage till I get the $ to work the swap, or I'll sell for $4K to at least recoup the cost of all the darn repairs and upgrades I've done to this car: new rack and pinnion, vented brake disks, brake system, custom intake, custom manifold and greedy double exhaust, wheels, yokohama tires, ferrari type airhorn, clear signals, A/C, new alternator, new rear axle bearings and drive shafts, drained and refilled transmission, engine, and coolant, custom oil pressure gauge, stereo, sony xplode speakers, clutch pedal assembly, manual brake pedal assembly, master and slave clutch cylinders, leather shifter boot, JDM short shifter assembly (Factory one), and car cover. I'll give the sweat and pieces of skin lost during the work on it for free. God darn this car frustates the hell out of me... I'll trade for a classic carburated cellica or corolla (70s) in good shape.

[Scorpo90]

Alternator $300, Distributor $280, Coil $60, Wires and plugs $140, Fuel Filter $20, been asked at the gas station "what year is your Ferrari?" Priceless...


Hesitation gone! Fuel filter and timing adjustment were the main culprit, full of sediments, my worry now is how much of it is in the gas tank, and where it is coming from, which I fear is rust dust, which means a new tank and/or fuel lines. The love never ends...

I still liked the dumb horse who thought it was a Ferrari...

[Scorpo90]

Ok new update, swapped the computer for an Ebay $10 replacement, it works a little better than mine, critical acion taken was to retard the ignition timing to 9-8 degrees, instead of factory recommended 10, big improvement, throttle response got better and no hesitation. I was able ti pin down the hesitation to lean conditions, thus why to retard ignition. So the teaching is not to be a retard and start buying parts like crazy, just retard your ignition timing.

[Scorpo90]

Next clean the O2 sensor out, the moronic computer in MR2 will not always trigger a faulty O2 sensor code. I know mine has to be cleaned every 6 months due to running a little richer as a result of a faulty fuel pressure vsv, that I don't have the $ to replace right now. Thank you.

[phoenixpower]

ebay replacements are denso and very cheap (20) please dont clean it, it will not help, you need a new one, they last a long time.

[Scorpo90]

Yeah noticed that it didnt help. I check Ebay cause last time I spent $70 on it from PepBs.. Thanks for the headsup. I'll post after.

[Scorpo90]

Learned a new trick from MR2.com when checking the O2 sensor, disconnect the EFI 15amp fuse, driver side of engine bay, within the black box where there's a positive terminal, is at the top, you wont miss it, disconnect for about 15 seconds, this resets the ECU. Disconnect the O2 sensor wire (1991) is one wire sensor, don't know if this works for other model years. Start car and drive for 30 minutes, make sure engine gets to normal operating temps. If the car does not hesitates and drives ok, but very rich, you'll smell it, then the problem is the O2 sensor, replace it. NAPA $50, 12051 I think was the part number. To check it just meter the resistance after the engine has warmed up, i should read between 0.1 and 0.8 ohms. optimum is 0.45 ohms. But if the 1st step works, just replace it. My next big project is getting a head job.... jejejeje factory valve seals become crap after 19 years of use, leakage damages the O2 sensor with sooth. I'll try to get a pre-fab head with custom rods and cams. I need to dig where I saw someone who does these for around a $1K.