Hesitation on acceleration

I am experiencing severe hesitation, poor acceleration, and atrocious fuel economy from my '98 V6 Tacoma. On initial takeoff (especially when the engine is cold) I may as well have stuffed it into fifth gear. At around 3k rpm, it stutters again. I have poured over this and other forums in search of any solution to this problem. It is not environment sensitive - I have seen many threads that mention this problem only occurring on hot days or in rain. At this point I have replaced:
- MAF sensor
- Spark Plugs
- Spark plug wires
- Fuel Filter
- Air Filter
I have seafoafed every thing possible (PCV valve vacuum line, crank case, through fuel system) I have even used the aerosol version in front of the throttle body.
I have fogged the engine as best I could for vacuum leaks.
A mechanic friend told me to pull out the o2 sensors and take a lighter to them (made no sense then or now) and nothing.
I paid the $100 to have a mech put it on a diagnostic machine and they quit looking after cleaning the MAF sensor (been there! done that! and came back in a day or two).
While investigating fouled MAF, I discovered that the little clips holding the air filter against the gasket were loose, allowing dust and debris to find its way around (I don't want to consider for how long). What damage could this cause?
It threw an engine code months ago (misfire cyl 2) and hasn't came back since the plug wires.
Shes got about 270,000 miles (its a taco, so what!?). Any ideas? next steps are TPS, o2 sensors, swap coils, replace intake boot to identify air leak... Rebuild... Engine swap...
It seems from what I have read that this isn't an isolated issue. Hopefully someone has found a solution.

 

Bad injector ?

 

Bad injector? Bad compression? Leaking head gasket? Possibly even a valve adjustment?

Believe it or not, even these trucks need a valve adjustment at some point, if you have never done it.

 

TPS sounds like a good next step. I take atrocious fuel economy as a sever overfuel,,causing a rich stumble.

Compression check is a good one also,,you might have a dead hole that the motor is carrying.

A bad coil could do the same thing.

 

I have considered the all to likely possibility of head gasket. All other signs (from what I have been told) like antifreeze in oil and visa versa are not present. In early stages of the problem, I noticed an odd sound that had me convinced it blew, like a steam engine tiss tiss tiss while driving on the interstate. What other symptoms? She leaks oil pretty good I know that, thinking it's from oil pan gasket and valve covers. A Compression test is probably gonna ruin my day but that is next direction I plan to go or possibly o2 since the last sea foam kicked out enough smoke to probably foul it.

 

In other threads, I ave seen repeatedly that replacing injectors did not solve anything (assuming that it's a similar problem). Never tackled a rebuild before, but from the sounds of it, it would likely remedy the problem and return the motor to its youth. What kind of fun would that be?

 

Mild overheating past its normal mark at any point in time, wether it be in primetime traffic or just cruising down the highway.

I have worked on many vehicles that indeed had a blown gasket to such a minor degree that it was causing a hesitation on acceleration. It does happen. The instance is rare when a blown gasket does not cause coolant loss, but just because it is rarely uncommon doesn't mean that its not blown to such an extent where it can cause a miss.

Given your mileage, I think it's time for an in depth check at your dynamic compression to make sure there is no air loss at the valves, as well as a cylinder leakage test anyways to ensure the head gasket nor the rings are leaking excessive pressure. Anything under 25% is normal due to their (the piston rings) openings from end to end.

I would also highly encourage it due to the amount of heat up/cool down cycles that engine has seen in 270,000 miles. I do not know if there are dissimilar metals being run on the bottom end vs. the top end much like GM vehicles of the 90's (cast iron block and aluminum heads) but if it is anything like that then the heat up and cool down cycles of the two different metals are different temps, which would further exacerbate the issue of hot spots along the heads, causing warpage along its mating surface to the gasket and ultimately its bottom end.

I know it sucks to do these kind of tests on a V6 with awkward or limited workspace, but if you want to get her back into running condition you really need to check those avenues to make sure you cover your own ass. While you are performing the compression test you should do the cylinder leakage test right after since you have it all apart. Kill two birds with one stone. I have never seen a fouled up o2 sensor due to sea foaming, however, if it has been a long time (I mean over 100,000 miles) since the last time you have touched upstream oxygen sensor(s) I would go ahead and replace that too. Those can cause you nasty gas mileage and a good amount of missing if they are lazy and sticking in voltage.

EDIT: One more thought. I see you have replaced your spark plug wires as well as the plugs. The coil(s) are an item commonly overlooked and I would not dismiss them at all. This one is going to be your call, but at some point you should look at them during your examination of the motor. Personally if I were in your situation given the higher mileage I would go straight into the heart because I want to make sure all of that is okay before even spending any more time on other items that might be okay. If you need help diagnosing the coils, let me know.

 

Thank you for your input. I apologize for the lengthy time before reply. I have since experienced a tps code. I replaced the sensor with a used one. No improvement, code came back, tried a different one. Same thing. I am currently using the original. Cleared the code about a week ago and it hasn't come back. i then swapped the IAC valve With one known to idle high - idled fine (go figure) I just performed a compression test. ALL cylinders were at almost exactly 98psi. No major carbon build up on any individual plug, but minor sludge around threads. I was going to borrow an ecu, but now I am not sure. A mechanic buddy who is not particular to tacos threw out the idea that perhaps the timing belt had slipped a lug... Idles ok, nice and low but a little wobbly. any further ideas?

 

98 psi? There is your problem. Your engine is wore out.

 

I would bet to guess if all cylinders are equal with that kind of compression, either the compression tester was bad or your rings are all equally wore out.

 

Swapped the motor, shes alive and roaring again!

Bingo. Even had issues with the brakes in the end, not enough vacuum to power the brake booster. Should have jumped into the heart of the problem to start.

 

Update the part that I replaced that corrected the idle fluctuation issue was the vapor sensor solenoid or sensor





While I was at the salvage yard i found a Tacoma (very rare to find one in a junk yard in OK) the motor was gone but there was a part I found on the drivers side inner firewall/fender with several vacuum lines running to it. I had replaced the o2 sensor, EGR Modulator, Vacuum switching valve, did a tune up, fuel line filter and some other parts I can't remember. But as the hesitation grew worse into a bucking horse at times and idle fluctuation became erratic. I decided for a few bucks to remove the part and give it a try. I think autozone was around 80-100$ for this EGR Vacuum Solenoid. But this was the part that stopped the idle jumping up and down and only a slight hesitation when going into second gear around 2-3k rpm remains. Its not like before though where the truck would just completely stop acceleration or at certain speeds just not go any faster unless I down shifted, let off the pedal and engaged while slowly accelerating to pick up speed . I think the other part that one of the vacuum hoses runs to is also a solenoid of some kind. I will replace that next and see if that solves slight hesitation problem.

 

We’ll I’ll knock on my wood 277,000 on mine and all valves were very in spec, with synthetic oil changed @ 3k. That doesn’t mean valves won’t need it one day but these trucks are righteous.

 

I would consider replacing the IACV and TPS with new parts. doesn't need to be toyota, NAPA parts will do. Used parts can be within spec and will not throw codes but the engine will still run poorly.