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Q: Low Compression on Cylinders 1 and 2 on 1999 Toyota Tacoma

Hi. I live in the Bay Area, CA and I have a ’99 Toyota Tacoma 4WD with a 2.7L 4-cylinder and 158,000 miles. I think I’m having compression problems. The mechanic I’ve been going to is not a Toyota-specialized mechanic – just a Mom and Pop place (all the work listed below was done by this guy except the idle air control valve). The check engine light has been on and his diagnosis is that cylinders 1 and 2 have low compression. His recommendation based on the age of the engine is that I get a new engine. So I would just like to have a second opinion before investing in that kind of expense. Here is a brief history which I hope will provide enough info for an informed opinion of the problem…

I bought the truck in 2004 from a contractor with about 90,000 miles on it. In hindsight, it probably wasn’t the best decision. The outside of the truck was pretty scratched up, so there’s probably a good chance he didn’t maintain it very well either.

I’ve taken decent care of it – usually changing the oil every 4K-5K miles, occasionally going to 6K if I forget.

The only major repair I had before was in March ’07 after it had sat unused for about 5 months (not sure if that was related). The idle air control valve had gone bad (causing high idling) and I had it replaced. A throttle body flush, air induction flush, and throttle body cleaning were also part of this repair.

The check engine light first went on about a year ago, but then it continued to cycle on and off – often staying off for days or weeks at a time – there were no other symptoms, so I didn’t worry too much about it. At some time after that, the idle started to stick a little high at times also (around 1,100 rpm).

In June ’09, after having an oil change, I noticed a very slow drip of oil for the first time. I took it back to the mechanic who said it looked to be coming from the oil pan. He said the new oil might be getting through a hole that could have developed a while ago which the older, thicker oil could not escape through. I believe the drip stopped after a week or two.

In August ’09, I had it emission-tested and it passed no problem.

In September ’09, I finally had the check engine light diagnosed which said that the throttle position sensor was bad. I had that replaced and the throttle body cleaned, and the check engine light stayed off after this. My mechanic recommended that after doing this work, I should have the sparkplugs and fuel filter replaced ASAP.

In October ’09, he replaced the AC/water pump belt, sparkplugs, wires, and fuel filter (immediately prior to this work, I put in a bottle of Lucas fuel injector cleaner, per his recommendation). It turned out the wires and fuel filter had never been replaced before! At this time he also did a compression check, I think because spark plugs 1 and 2 showed signs of mis-firing (slightly blackened). On the first check, the pressure was:
#1 – 70 PSI
#2 – 60 PSI
#3 – 180 PSI
#4 – 170 PSI

He sprayed some fluid in, and rechecked 1 and 2, which had then raised to:
#1 – 80 PSI
#2 – 100 PSI

At this time, the check engine light was still off, but about a week later, it came back on. He diagnosed it again. This time, the computer indicated a “random misfire” on cylinders 1 and 2. This brings us up the present and his final recommendation that the engine be replaced because either the piston rings are bad or something is possibly cracked.

I haven’t noticed any drop in power or any other symptoms, although I think my fuel economy is down. I switched from regular to premium gas for a few tanks to see if that would make a difference, but I haven’t noticed anything. On the last tank, I was averaging about 14 mpg, although it was mostly city driving, often with heavy loads – but that still seems low.

The only noises I’ve noticed are the same ones that have always been there since I bought the truck:
-knocking for the first 20 seconds or so after starting on very cold mornings
-occasional squeaking – maybe a belt?

The check engine light is still on, and a few times it has started flashing, seemingly related to high revving – either accelerating or engine braking.

So thanks so much for your time! Any insights to the problem and possible solutions would be greatly appreciated! If it seems likely to be a major expense (in the $1,000s), I’m considering selling the truck as is and upgrading. But it would be nice to keep it if a relatively inexpensive repair could remedy the problem. I would just like to know if my mechanic’s diagnosis sounds accurate and if his recommendation to replace the engine sounds like the best solution, or if there is a cheaper option.

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The preferred test for determining the cause of low compression is a "cylinder leak down test". This is where the engine is rotated until the valves are closed in #1 cylinder, then air pressure is added to the cylinder via the spark plug hole, then you check for where the air is leaking out. Repeat on all cylinders.
I would want to have this test performed then make a determination. There are many shops in your area that should be able to perform this test.
You don't say how many miles you have on the engine, but if the cylinders/rings re worn, then a used or re manufactured engine may be your best choice. There are many options, so shop around a bit.
i have the same vehicle, after 1yr of "mis-firing cylinder, need tune-up ect) stranded in san antonio, i noted there was a complete 90 degree kink in the flimsy air inlet hose that goes to the air filter,next to the radiator, i cut 2 inches off, never seen that stupid light again.
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