03 Prius intermittent engine stumble as if it's trying to start

Hello fellow Prius owners. Great forum! I have an issue with my 2003 Prius that I can't figure out. Maybe someone on here has experienced this. I think it might be an ECU calibration issue. I don't have a Toyota scanner so there isn't anything I can do about reprogramming the ECU to the latest calibration. Is there an ECU calibration update/s or updated replacement ECU to fix my issue listed below?

Ready mode, after driving 10 miles or so, engine warmed up, at a stop (engine off/Ready). The engine feels like it's attempting to start and stumbles once. After a few seconds pass it stumbles again and repeats a few more times. The stumble resembles the exact moment of a slight misfire in a running engine. There are no warning lights nor MIL/DTCs when this problem occurs.

In the past I have retrieved pending P0300 (random misfire detected), p0301, p0303 etc. when scanning the computer to check on things. I have never felt a misfire while the engine is idling or under load.

I bought the car with 118,000 miles on it.

Service history: I was a Ford/ASE certified mechanic for 8 years. I perform all of the work myself. 20k miles ago I replaced the spark plugs with NGKs (set gaps to OEM spec) for routine maintenance. The old plugs looked in almost new condition. New 12v battery, Optima Yellow Top with conversion kit. Replaced transmission fluid with proper toyota OEM spec fluid, cleaned the pick up screen and magnet. Properly cleaned/oiled K&N air filter. Replaced inverter coolant, coolant pump with the updated part due to no coolant flow, DTCs, and following the TSB. I replace the engine coolant once a year. Regularly change the engine oil and filter with 5w30 oil, making sure not to over fill the crankcase. I had the dealer perform the steering shaft bolt recall.

Previous owner service history. They had the D/S front wheel bearing replaced, and the catalytic converter and oxygen sensor replaced under warranty. They were in a minor front end collision and the radiator, condenser, core support etc. were all replaced. I found a damaged radiator hose neglected to be replaced after the accident. The hose was gouged slightly from the drive belt rubbing on it from being pushed into the belt.

I usually run 87 octane from Sunoco. Last tank of gas I added a bottle of Techron fuel system cleaner before fill up. I just performed a fuel injector cleaning with my OTC specialty tool using 50% Seafoam 50% 87 octane fuel. Tested fuel pressure at time of service 48psi. Regularly clean the throttle body. Just performed an induction cleaning 1/3 pint Seafoam using OTC tool to central intake vacuum inlet.

I did all of the cleaning services for preventative maintenance. With higher mileage and no replaceable inline fuel filter. Why did Toyota not install a serviceable inline fuel filter on this model Prius? Has anyone fitted an inline fuel filter to their Prius? I'm considering doing this either in the engine compartment just in front of the fuel rail or near the tank.

My driving is low mileage short trips usually under 10 miles round trip per day. The MPG has dropped from the mid 40s to mid to upper 30s. Because of my low mileage city driving I'm concerned about excessive carbon build up. This is the main reason I did all of the cleaning services recently. I can't figure out why the fuel economy has declined so much.

Does anyone have experience and or information with adjusting the valves on this engine, and at what mileage it should be performed? I'd like to get the Toyota service manual instructions for this procedure if possible. Any help is greatly appreciated. Thanks to everyone in advance.

Below is some information about my Pruis that may help with determining ECU calibration version, diagnosis and or replacement parts.

Current Mileage: 175,300
I believe it's the original engine, transmission, inverter, and Hybrid battery.

Door Jamb Label Info:
10/02 Prius
VIN: JT2BK18U13007727
No 661
Model: NHW11L-AEEEBA
C/TR: 1C5/FC95
A/TM: 01A/P111

Behind Glove Box Engine Control Label Info:
175300-5060 12V
89661-47054
1NZ-FXE

Under Hood Engine Label Info:
3TYXV01.5LH1
21080
15V1GLHE
1NZ-FXE

 

Hello Bob,

Thank you very much for your quick reply, valuable information, and suggestions. Early Tuesday morning I inspected the throttle plate by hand for a sticking condition with the engine cold and warm. It was about 50°F outside. The throttle plate moved freely in both cold/warm conditions. I cleaned the throttle body last Sunday along with performing the fuel injector, and induction services. The problem remained, however after the servicing and additional driving it does seems less pronounced.

I checked again for pending codes with my INNOVA 3130 scan tool. There were no DTCs. During my test drive I monitored the live data/pid values that were available. I noticed during certain times at idle there was a 50°F difference between the IAT and the ambient air temperature (111°F IAT, 61° ambient). I expect the IAT to be warmer than ambient but that much of an increase seems to be on the high side. Is this a normal characteristic for the Prius? I'm interested to compare this reading with yours to see if it's similar. I duplicated the concern once, but it was a less harsh set of random stumbles.

According to my scanner the ECU goes into open loop each time it turns off the engine. During these conditions: (Ready) mode, engine off, while in Drive, I monitored the pid values for any readings that indicated the ECU was attempting to start the engine. Unfortunately when I felt the attempted engine start/stumble there were no changes in the pid values.

I inspected the IAT sensor element. It was covered with a thin layer of black carbon/dirt. I cleaned it with isopropyl alcohol soaked q-tips followed up with mass air flow sensor cleaner. It now looks good as new. I definitely overlooked cleaning the MAF sensor. I didn't find any oil contamination on either of the two MAF elements. I cleaned the MAF sensor and connector pins. Thank you both for bringing this to my attention.

I read over both of the guides you linked me to. Afterwards I cleaned the throttle plate again (engine off) with MAF sensor positioned to the side. This time focusing the spray solvent around the pivot points while moving the throttle plate. I followed up with lightly oiling the pivots. I inspected all of the throttle body vacuum inlets for clogs. They were all open. I cleaned them anyways with solvent and a brush just to be thorough.

The before/after MAF pid values didn't seem to be affected much by cleaning the MAF sensor. This doesn't mean it didn't resolve the issue though. The drivability has improved slightly upon cleaning both the IAT sensor and MAF sensor. Below are my recorded pid values in "Ready" mode.

Shifter in Drive, AC off: Open loop, Eng RPM 0, vehicle speed 0 MPH, spark advance 5°, ECT 183°F, IAT 91°F, ambient 56°F (display), MAF lb/min 0.009, TPS 14.1%, STFT 0, LTFT 8.5%

Shifter in Drive, AC off: Closed loop, Eng RPM (1180-1213), vehicle speed 0 MPH, Spark Advance 17°, STFT (1.5-8.5%), TPS 15.2%, ECT 188°F, IAT 91°F, Ambient 56°F, MAF lb/min (0.457-0.511), 02S B1 S1 (0.020- 0.800V), 02s B1 S2 (0.810-0.855V)

Shifter in Park, AC on (compressor cycling on/off): Fuel sys CL, Eng RPM (960-1218), ECT 188°F, IAT 91°F, Ambient 56°F, MAF lb/min (0.244-0.621), STFT (1.5-8.5%), LTFT 8.5%

Shifter in Drive, AC on (compressor cycling on/off): Fuel sys CL, Eng RPM (1185-1236), vehicle speed 0 MPH, ECT 188°F, IAT 100°F, Ambient 56°F, MAF lb/min (0.383-0.563), STFT (1.5-8.5%), LTFT 8.5%

I have sense driven the vehicle several times for about 30 minutes each trip. I couldn't re-duplicate the concern. I'd love to say that it's fixed, but it's too soon to say for certain. The concern has always been intermittent. It usually happens while sitting in traffic, so I believe it is or was temperature related.

I'm going to perform more test driving before removing the throttle body and TPS for servicing (following Hobbits guide). I'm trying to do one repair attempt at a time to see what fixes the issue. I have a good feeling that cleaning the MAF sensor will at the very least improve my fuel economy once the ECU has relearned.

Bob, If you get a chance to record and post your pid values under similar conditions for reference I'd greatly appreciate it. Unfortunately I don't have the Prius factory service manual as of yet to reference pid specs. I'm going to attempt to capture/record the pid frames with my scanner during future driving.

Thank you both for your wisdom and assistance. I appreciate your excellent write-up. I'd also like to thank Hobbit for his write-up. They were both helpful.

I knew that Toyota had put some type of fuel filter located in the Prius tank. Every fuel pump I have ever replaced had an in tank filter connected to the pump along with a replaceable in line fuel filter. Does the Prius have the same sock type filter that most vehicles have attached to the pump, or is it of better design and quality? If it's the same cheap type, I'm going install a replaceable inline filter. I'd like to protect my fuel injectors from clogging and having poor spray patterns.

 

Bob,
Thank you. Good luck with your laptop repair.

Hello Chap,
Thank you for the additional info on valve clearances. It almost sounds like I shouldn't even bother checking clearances if your tolerances were mostly within spec at 200k. There are so many variables though from one engine to the next. The Prius engine obviously experiences less wear and tear than a non hybrid vehicle without having to idle so much and the lower RPM range. I wonder how long I can make this car last.

Years ago when working at a Ford dealership I recall using feeler gauges to measure OHC/DOHC valve clearances. I'm sure a good dial indicator set up works too. Checking valve clearances was not an every day task though. Once I get my hands on a service manual I'll likely use whatever method they recommend.

Hi Patrick,
Were your valve clearances within spec? Do you recall the approximate mileage on the engine?

I remember reading in some Toyota Prius manual that a basic preliminary inspection for valve clearance was (Listen to the engine at idle for a noisy valve train, and also to inspect for a rough idle.) If neither of those were an issue, no valve adjustments were needed.

I personally don't feel confident checking things this way. Maybe this is Toyota's method for warning us that the valve adjustment procedure on this particular engine is very involved, and best to avoid unless required for repair.

My valve train doesn't sound particularly noisy but it isn't exactly quiet either. The engine idle is fairly smooth for its mileage. This is one of those times it would be helpful to have another Prius that's known to be within specs for comparison. If I remove the valve cover to check I'll likely end up having to special order the correct thickness shims along with whatever else is needed to complete the job. Sounds like there would be at least a few days of down time for parts. I'll have to plan this project ahead of time.

 

Bob, thank you for the recording. I noticed that recording was for your 2010 Prius. The next time you get a chance could you please take a recording using your 2003 Prius under the following conditions: normal engine operating temperature, gear selector in Drive, engine idle, 0 mph, AC on or heat on (to keep the engine at idle). Just let me know wether you turned on the AC or heat so I know how to compare the results. The conditions are important because it's when I experience the issue.

Chap, thank you for the details. I already have a few bloody knuckles from removing the wiring harness to access the PCV valve today. The last clip on the plastic wiring harness cover did not want to release. I inspected the throttle body after test driving it and the underside of the throttle plate already had engine oil baked on it. I haven't noticed this before. I understand some oil residue in the intake is normal but this seems a bit much. Seeing how I just cleaned it spotless I was not expecting to find this after 20 miles of driving. I'll order a replacement PCV valve but in the mean time I'll clean it and see if it reduces oil intake.

The valve adjustment sounds increasing fun the more details I read about it. I don't plan on replacing the engine any time soon. I'll drive this one until it dies. I love how the engineers make servicing their vehicles so user friendly. If they don't already they should get several mechanics input during the design process. Even if the parts are costly, Toyota would make more money selling a larger quantity of parts if they made their engines easier to work on. Not many people are going to pay a shop $800 to have their valves adjusted unless it's causing major drivability issues.

Dealer mechanics will try to avoid selling these types of jobs if they are so difficult and time consuming even for them with every special tool. This particular job sounds like the mechanic would get paid less than the amount of time he actually spends working on the car. If that's the case during some of the repair he would be working for free. Got to hate flat rate. I wish all mechanics were paid a good salary because their job is already difficult enough.

 

Yes, you are correct. The problem does occur when the engine is off at a stop while in D. The reason I requested the data while the engine is at idle is for additional comparison. I think it would help troubleshoot the issue, looking for a sensor that is on its way out etc. When the engine is off the ECU pid values I have access to view are displayed, however they are all at rest. The ECU is in closed loop, and the pid values are at their lowest KOEO key on engine off state. With none of the values active there aren't any ranges to view. You're right I should probably compare both conditions, especially with the engine off to at least see if my pid values base lines are similar to yours.

I connected a vacuum gauge to get a better idea on the condition of the engine. At idle the vacuum readings were steady between 21-22 in HG depending on how the ECU was adjusting things, and as low as a steady 16 " in HG with the AC on Max. I figure if the issue is the engine trying to start, the gauge will show a reading as it's pretty sensitive. My generic scanner on the other hand is likely too slow to record it.

While monitoring live data with my CanOBD2 3130 scanner the values will occasionally go blank for several seconds. I called the manufacturer today, and they immediately blame it on my vehicle's network. They instructed me to try it on another vehicle, automatically assuming I owned a 2nd vehicle with OBD2. I highly doubt my Prius has a network issue, from my experience that is extremely rare.

I'm finding that more and more companies aren't standing behind their products. Manufactures claim to offer tech support when in reality all they really do is place the blame on anything but their products. They are too quick to blame the vehicle's network just because the car has an issue doesn't mean it's related to the multiplex network. I do not recommend INNOVA scanners. They work just ok for retrieving and clearing codes, but are too slow for long term monitoring and recording pids.

When you manually trigger the scanner to capture/record data it takes several seconds before the scanner even responds and finally displays a wait message that it's recording data. There's too long of a delay which makes the user question if the scanner even got the command. This is frustrating as you're driving down the road messing with a scanner. It's already dangerous enough operating a scan tool as you drive down the road.

If only I could magically turn my NGS into a Toyota dealer scan tool. To the best of my knowledge my scanner only allows me to view a limited selection of ECU pids. I don't have access to any of the other ECUs that you mentioned are on the 03 Prius. I really need to get a better scan tool. Currently I can't afford one. The correct Toyota scan tool would probably cost more than my car's resale value.

 

Bob, thank you for the data, and information on the scanners. I'll use this info to pick out a better scan tool.

My Prius is in stage 4 of the warm up operation when the symptom occurs. The problem is that while the engine is off, at a stop, the engine jumps/shakes for only a fraction of a second, pauses and repeats a few time. The engine remains off as if it didn't need to self-start in the first place. It's as if the start up attempt is being cut extremely short, and the ECU changed its mind and decided it didn't need to start after all.

Sorry I'm not good at explaining the issue. Imagine if the engine being off was actually at a smooth idle, and the attempt to self-start was a slight stumble/misfire. Only the brief stumble/misfire is what I'm feeling behind the wheel. This happens fast and intermittently.

The fuel injector cleaning seemed to help reduce the harshness of the stumble. I disconnected the fuel line and ran the cleaner directly through the fuel rail and injectors. I temporarily blocked off the fuel supply line with an adapter. There was good fuel pressure, volume/flow. I used the fuel pump to fill the OTC cylinder with 50% fuel.

The MAF and IAT sensor cleaning improved drivability by smoothing out acceleration and slightly increased engine performance. The reason I'm mentioning this again is because you could be correct about a fuel injector performance issue.

After looking at your 03 Prius data I noticed a 19.8°F rise in IAT. My IAT had a 55° F rise. Is your IAT/MAF sensor and air intake a factory set up, and location on the engine? Our ECT readings are the same, and ambient air temperature was similar. I think my IAT readings are too high. I'm going to compare it with my DVOM temperature probe reading. I'll pull the spark plugs today, measure the gaps and look for any indicators.

I don't think turning the AC off prevents the problem from occurring. I don't use my AC too often. I always try avoid turning it on to save fuel during my mostly stop and go driving.

You have an interesting and excellent idea. I'll try to locate and pull the "CLR MG RELAY" and see what I can find. I don't think the relay locations are mentioned in the owners manual only the fuses. Maybe the relay/fuse box cover identifies this relay.

Thank you for taking your time to help me out with this. I greatly appreciate it.

 

Patrick, that is good information to know. Thank you. My fuel economy is in the low to mid 30s . I do mostly stop and go driving through town. That's likely the biggest contributing factor to my lower fuel economy. I used to get around 40-45mpg. My Prius hasn't always exhibited this behavior from when I purchased it used several years ago.