Rough Idle/misfire 1998 2.7 Liter 3RZ-FE (Can't figure it out)

Hey All.

My 1998 2.7 liter has had a rough idle and I can't seem to figure it out after a month or two of diagnosing. I've taken it to two shops and neither could confirm or find the issue. Looking to this community for help if you can.

The truck is a 1998 Toyota Tacoma 2.7 liter 4 cylinder (3RZ-FE). It's a 5 speed, 4x4 with 180k miles on it. I got it about a year and a half ago from a friend and it's been very well taken care of.

It started having a rough idle at cold start on especially on cold mornings a few months ago but wouldn't happen every time (video of rough idle). But lately it has been getting worse and worse. What happens is after I turn it over at cold start the idle will go up to about 1,100 RPM and then fairly quickly drop to about ~650-700 RPM and then start to shake intensely. This will go for a long time until I start to drive. Sometimes it won't start to shake/misfire until about 1-2 minutes after driving. The roughest idle I've experienced with it was when I parked it outside overnight and started it in 28 degree weather. But, some days I will do a cold start on a semi-cold day and it will stay at ~1,100 RPM and then the idle will not drop at all with no shaking or noticeable misfire.

I took it out late last night after 20 hours of sitting and the engine was shaking very intensely and sounded like it was getting to the stalling point at about ~500-600 RPM. After driving another 10-20 min idle returned to about 1,100 and ran fine there, but the idle was wavering some but staying high.

After a long 25-30 minute plus drive the engine will run mostly fine. No issues with acceleration or loss of power in higher RPM. It seems to be in the first 15 minutes or so the issue really stands out. It isn't throwing any codes at all and has had no check engine light. It blows no blue black or white smoke, I haven't needed to add any coolant in the past year, and no milky substance in oil cap..

Two shops have looked at it...

Shop #1: Toyota Dealer/Official Toyota Mechanic:

• Claimed the misfires were in cylinder 3+4 and saw coolant in each with borescope
• Wanted to replace head gasket for $5,000
• Could not confirm leak was source of misfire (I did not trust diagnosis because of this)

Shop #2 (for second opinion)

• Did 3 separate borescope tests and did not see coolant in any of the cylinders
• Could not find source of misfire

I personally have inspected or replaced:

• Spark plugs (Replaced. Old ones had no signs of excessive wear. All had consistent wear pattern)
• Wires (Replaced)
• Ignition Coils (Replaced)
• Coolant Temp Sensor (Replaced)
• Idle Air Control Valve (Replaced)
• PVC Valve (Replaced)
• Cleaned Mass Air Flow Sensor
• Did carb cleaner vacuum leak test (sprayed around intake parts and vacuum tubes)
• Block test (For blown head gasket. Passed)
• Ran fuel system cleaner through (thinking of doing Seafoam next for injectors)
• New battery within the last year

I am incredibly frustrated at this point. Part of me thinks it may be a fuel and/or air mixture issue or sensor. If you think you can help with this I would be so incredibly appreciative. Thanks! -P

 

My 2.4 was doing the same and new denso injectors made it 90 percent better, but now i think a new iac will finish off the fix

 

What I'd do before throwing anymore parts at it is drive around with a scanner hooked up..

Watch the fuel trims.....the maf numbers....throttle pos...open/closed loop....real engine temp.....ect...

On the more expensive ones...you can see if the egr valve is switching on and off.....

Look for some wacko engine sensor out of spec.....one by one.....


Simplest thing you can try is clean and check the egr valve....they get carboned up and have to do the chimmy sweep action.....

If you remove that .....expect those two gaskets to be toast....

That pcv line that goes into the throttle body can carbon shut too...right where it screws into the TB.......

Same with vac lines...leaky ones cause weird idle probs......smoke test is super simple....

 

Similar situation here with a 1996 at 300k+ miles. Initial symptoms were subtle misfires only at idle for a month, then after a long road trip of normal smoothness, it badly misfired and "Check Engine" lit up. Replacing all four fuel injectors cured the rough idle when warm but it now stumbles on cold starts (have to hold pedal to keep revs up) and misfires randomly, usually under load uphill but sometimes on level ground in 3rd or 4th gear, not speeding.

I've done everything in your list but these so far:

• Spark plugs (Replaced. Old ones had no signs of excessive wear. All had consistent wear pattern) [mine only have 20k miles, might check them]
  
• Coolant Temp Sensor (Replaced)
• Idle Air Control Valve (Replaced)
• Cleaned Mass Air Flow Sensor [actually replaced mine, plus the IAT on the air intake box]
• Did carb cleaner vacuum leak test (sprayed around intake parts and vacuum tubes)
• Block test (For blown head gasket. Passed)

I replaced the fuel filter, too, which is a good idea regardless of "lifetime" claims. I also suspected the hard-to-each ECT, but if that's what's sending temp info to the dash display, mine seems normal. You'd think it would read high if it triggered too little fuel at cold starts.

Replacing both oxygen sensors is my next goal but they seem OK on OBD scans, and wouldn't seem to affect cold starts since they function when hot. A subtle vacuum leak is probably the toughest misfire cause to detect, but rough cold starts are a big clue to something. It drives smoothly otherwise, so we don't have exactly the same issue.

Many mechanics semi-blindly replace parts until one fixes it, but of course that's a money-pit if your own mind is more sophisticated for troubleshooting. I only use mechanics for things I lack tools or a lift to do. They're in a hurry and tend to lack time for fine points, plus they'd have to drive it daily for a full concept.

 

P.S. That paragraph got me thinking that a vacuum leak might be self-sealing after some metal part expands enough from heat. I didn't try driving mine beyond a parking lot when it got a bad rough idle, since unburned gas to the catalytic converter can wreck them.

 

The easiest thing you can do to your old friend.....is give em new vac hoses...
all thru.....
Very good bang for the buck.....and one thing to cross off....

rotten pcv grommet is a big culprit....

 

Are you leaking coolant ? Is there brown muddy build up in radiator ?
Is exhaust blowing ton of white smoke ?
Lastly, rent borrow or buy a maddox combustion kit and test the radiator for combustion gas.
classic symptoms of head anti freeze leak is rough start, and rough idle. It usually runs fine once
rig is warmed up.
Simple and straight forward. If your getting combustion gas in the radiator, most likely your head gasket is leaking.
Last ditch if you still will not believe it, is to try a leak down test with a radiator pressure test. If you have radiator tester hooked up at 17 lbs,
hook up leak down tester to suspected valve and turn on air. Watch radiator gauge. It pressure goes up, your head gasket is
compromised.


I do not get why this is such a mystery to you. You need to figure out if your head is leaking and if you want to extend 'its life for
6-12 months with an additive. If it ain't the head, then proceed from there. Diagnosing a head is pretty basic.