High compression motor, Hot Weather, Low or bobbing idle

[suprunner]

Hi all, 

 

 EJ251 block/EJ25D heads in '98 OBW, 5MT

Recent work done in the past year:

Plugs (ngk)

wires (ngk)

Timing belt kit (oem)

Timing cam gears (oem)

Cleaned and swapped out IACV

Swapped out MAF 

Swapped out TPS

New Fuel Pump, Strainer, Filter, Pressure Regulator

New OEM Oxygen sensor

Compression test cylinders 1-4: 214, 205, 225, 209 (unsure if these numbers are good for a High Compression build, it is consuming some oil. Maybe a half quart to full quart every few months of mostly short trips and some highway)

 

I live in Las Vegas, and the temperatures are now at the usual hot. When city driving during the heat of the day, the engine will idle very low (has approached stalling, entire car shutters/shakes) and will often bob up and down +/- 100 rpm. It seems to be at its worse always when the fuel tank is at or below half-filled. If I use the A/C it helps with the issue because of the idle-up function. I've checked for vacuum leaks with a propane tank, and can't find anything. If I unplug a vacuum line, the engine idles up. Upon startup for the first 5 minutes, the exhaust smells very rich. 

Should I be looking to test next? The valves are kinda loud, should I do a leak-down test?

When I press the accelerator pedal, from 1800 rpm to 2500 rpm, the motor behaves as if it is unbalanced. Anything above 2500, it is smooth and responsive. 

I am using an air intake from a '96 legacy wagon... is that an issue? Too restrictive? 

 

Any help would be appreciated. 

 

Greg

 

 

 

[GeneralDisorder]

What are your fuel trims? What is your MAF G/s and voltage at idle? 

GD

[suprunner]

1 hour ago, GeneralDisorder said:

What are your fuel trims? What is your MAF G/s and voltage at idle? 

GD

GD, for Fuel trims and MAF G's/voltages can I get that information through a computer like those found at the auto stores? Also, what are MAF G/s? Is that grams per second? Do I want to try and read that information when the motor is presenting the symptoms, or just at any time? Have you had any luck with the bluetooth OBD II readers? 

 

Thanks,

Greg

 

https://www.amazon.com/gp/product/B00652G4TS/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B00652G4TS&linkCode=as2&tag=bluedriverweb12-20&linkId=5923cdc63008d567562f7afe37939bf1

Edited July 3, 2020 by suprunner

[GeneralDisorder]

You need an OBD-II scan tool. 

MAF grams per second. 

GD

[suprunner]

11 hours ago, GeneralDisorder said:

You need an OBD-II scan tool. 

MAF grams per second. 

GD

The OBD-II scanner from the local auto store couldn't show me values for my A/F ratios. For the MAF g/s, at idle stayed at about 3.02 g/s to 3.4 g/s.  There wasn't much of a change there when the car started to drop the idle. Alternator output is in the mid 13s on idle, and just below 14v at about 2000rpm. I noticed that the spark advance was 15 degrees at idle, until the idle drop/bob happened, that's when the spark advance jumped to 21 degrees, and there was one time when the vehicle speed sensor registered as 45mph. When I first connected the scanner, I slowly depressed the accelerator pedal (with the key in the ON position only, not running), and I consistently couldn't get the TPS to read over 92% when fully depressed, and without any floor mat underneath it. Does that mean anything, or is it just part of the governor system? I will track down an OBDII scanner that can show me A/F ratios. What values should I be looking for? Is this information in a FSM? 

 

Thank you,

Greg

[GeneralDisorder]

You need to be looking at long term and short term fuel trims (LTFT bank 1, STFT bank 1). It's not going to show AFR. It has a narrow band O2 sensor which is going to dither between 0v and 1v - typically they will cross 0.5v every second or two and bounce between 200 and 800 millivolts. This is called closed loop. The fuel trims are what the ECU is adding or subtracting to accomplish closed loop. 

Timing jump when idle drops is a result of the stall saver logic - adding timing speeds up the engine. 

No. None of this live data interpretation is in the service manual. It's a skill you learn (sometimes in school) and that is honed over many years of experience and tuning. 

GD

 

[1 Lucky Texan]

running premium fuel? even so, you may need some octane booster.

[suprunner]

On 7/3/2020 at 2:36 PM, GeneralDisorder said:

You need to be looking at long term and short term fuel trims (LTFT bank 1, STFT bank 1). It's not going to show AFR. It has a narrow band O2 sensor which is going to dither between 0v and 1v - typically they will cross 0.5v every second or two and bounce between 200 and 800 millivolts. This is called closed loop. The fuel trims are what the ECU is adding or subtracting to accomplish closed loop. 

Timing jump when idle drops is a result of the stall saver logic - adding timing speeds up the engine. 

No. None of this live data interpretation is in the service manual. It's a skill you learn (sometimes in school) and that is honed over many years of experience and tuning. 

GD

 

Hi GD,

 The STFT1 ranged from 2.3 to 10.2 while I was idling. The LTFT1 stayed at 15.6 the entire time. Thoughts? Also, when connecting the computer to the OBDII port, and its startup, the "EVAP" light was flashing on the computer, and in red. Should I be looking at the emissions system(s)?

 

[suprunner]

On 7/3/2020 at 3:57 PM, 1 Lucky Texan said:

running premium fuel? even so, you may need some octane booster.

I run a mixture of 100 oct, and 91 oct. The 100 octane is $8 a gal! I usually put in 4 gallons of the highest, and then the rest at 91 octane. I've not noticed any difference in performance if I just do 91. The low idle/bobbing is the worst whenever the gas tank is at or below 1/2, especially if I drive somewhere and park for a half hour or so. If there is an evap issue, is this a symptom? I never hear a positive/negative pressure release when I undue the gas cap... Sometimes I hear bubbling in the gas tank upon startup. 

 

Greg

[GeneralDisorder]

You have a serious lean condition. The two trims added together should be no more than plus or minus 5%. 

Here's the possibilities:

1. MAF is under-reporting airflow. Seems unlikely considering the reading from it. 

2. O2 sensor is skewed and reporting a lean condition. Or you have a constant misfire - which is interpreted as a lean. 

3. Fuel pressure is low or injectors are partially plugged. Resulting in unpredictable injector flow and the ECU is compensating by raising the fuel trims. 

Post a video of the engine idling. And do a kill test of every cylinder by unplugging the injectors one at a time. 

GD

Edited July 5, 2020 by GeneralDisorder

[suprunner]

6 hours ago, GeneralDisorder said:

You have a serious lean condition. The two trims added together should be no more than plus or minus 5%. 

Here's the possibilities:

1. MAF is under-reporting airflow. Seems unlikely considering the reading from it. 

2. O2 sensor is skewed and reporting a lean condition. Or you have a constant misfire - which is interpreted as a lean. 

3. Fuel pressure is low or injectors are partially plugged. Resulting in unpredictable injector flow and the ECU is compensating by raising the fuel trims. 

Post a video of the engine idling. And do a kill test of every cylinder by unplugging the injectors one at a time. 

GD

Hi Rick,

 I'll get a video of it idling tomorrow. I'll try to get it during the hottest part of the day to see if it shows everything I'm trying to describe. I've had this motor build for about 4 years now, with the same symptoms, it's just that I'm now living in the desert heat so the symptoms are always present. The symptoms followed the motor when it was transplanted into the OBW that I'm now driving. The only electronic components that are still on it from the beginning are the injectors, cam sensor and crank sensor. The MAF has been swapped out between two used OEM units, and an AC delco reman. Ignitor and coil are not original either, but there was no change in symptoms. About 4 months ago I had the injectors out, and I cleaned out all the filters, and soaked the tips in some isopropyl alcohol. There was no change after installation. 

Thanks for your time,

 

Greg

[suprunner]

On 7/5/2020 at 12:51 PM, GeneralDisorder said:

You have a serious lean condition. The two trims added together should be no more than plus or minus 5%. 

Here's the possibilities:

1. MAF is under-reporting airflow. Seems unlikely considering the reading from it. 

2. O2 sensor is skewed and reporting a lean condition. Or you have a constant misfire - which is interpreted as a lean. 

3. Fuel pressure is low or injectors are partially plugged. Resulting in unpredictable injector flow and the ECU is compensating by raising the fuel trims. 

Post a video of the engine idling. And do a kill test of every cylinder by unplugging the injectors one at a time. 

GD

Sorry for the length, it was hot outside and the engine bay was quite warm, I was fumbling on touching things. There are a couple instances where the motor does stumble on its own without me doing anything. I tried increasing RPMs to show the odd range of motor unbalance, but I'm not sure it shows well in the video. Unplugging injectors 1, 2, and 4, caused the motor to really bog and shake... Unplugging injector #3 had minimal change to the idle. Change it out?

[GeneralDisorder]

Since they are side feed injectors you can simply swap them around. Swap 1 and 3 and see if the kill test lack of roughness moves to #1. 

GD

[1 Lucky Texan]

try running with the gas cap off/loose? that might condemn or exonerate the evap system.

Edited July 7, 2020 by 1 Lucky Texan

[suprunner]

18 hours ago, GeneralDisorder said:

Since they are side feed injectors you can simply swap them around. Swap 1 and 3 and see if the kill test lack of roughness moves to #1. 

GD

So, I guess my initial evaluation wasn't what I had thought. I swapped #1 and #3 as you had suggested, and there was not change in performance. Unplugging both during idle produced the same effects. After clearing the codes, and taking for a spin, I still experienced the instances of low/bobbing idle. 

[GeneralDisorder]

Have you checked fuel pressure?

GD

[suprunner]

21 hours ago, GeneralDisorder said:

Have you checked fuel pressure?

GD

Not since I put in a new pump/strainer/filter about 4 months ago. You want me to check it? Also, if these injectors have many many miles on them, is it worth replacing them? If so, do they need to be OEM, or will something from Rock auto work?

 

Greg

 

[GeneralDisorder]

I've never had to replace NA injectors. I wouldn't start there. 

And yes - check your fuel pressure. 

GD

[suprunner]

On 7/8/2020 at 7:35 PM, GeneralDisorder said:

I've never had to replace NA injectors. I wouldn't start there. 

And yes - check your fuel pressure. 

GD

Finally got to checking fuel pressure. 28 psi at the lowest when the FPR is still plugged in, and goes up to about 36/38 psi when FPR is unplugged. No significant change in fuel pressure when I open the throttle to the normal hesitation range of 1800-2500 rpm. But the fuel pressure jumps to 30 psi when the motor begins to do the hesitation/low-idle/bobbing during idle. I also recently had the CEL come on while driving out of a parking lot at the hottest part of the day. Codes were misfires for all cylinders. CEL went away after a day, then came back while on the highway. Also, it continues to smell rich upon startup. I've also noticed that when I'm coasting with foot off the gas, but in gear, that once I reapply acceleration there's a pop sound that comes from the exhaust at the front of the car. Sound reminds me of engaging the snare mechanism on a drum. 

 

Greg

Edited July 18, 2020 by suprunner

[suprunner]

I've found this. I'm assuming that I should start here. The MAF has been cleaned, but it is old, and all my symptoms are under the MAF column..  Is there a diagnostic section on each of the components here? Same for TPS...