I have a 1998 Toyota 4Runner V6 that has been to 3 different shops, and nobody can figure out exactly what is wrong with it. To start, over 2 months ago my timing belt skipped, or jumped, time. The motor shut off as expected. After taking to Toyota, having the timing belt and tensioner replaced, I noticed that my vehicle was running sluggish and a little rough. I took it back to toyota to double check the timing belt to make sure it wasn’t “off by a tooth”, and perform a compression check. The timing belt was “dead on” and the compression was “within range”, and the vehicle was not “throwing any codes”. The technician also check the throttle body and cleaned up some carbon build up. This didn’t fix the issue. After personally cleaning the Mass Airflow Censor and replacing spark plugs (to eliminate possibilities), I took back to Toyota, for them to determine it was the transmission. Like most dealerships, they wanted to replace the entire tranny (which was more than I could afford) rather than replace the parts. I decided to take my 4Runner to a transmission specialist, and was guaranteed “it’s definitely not the transmission.” The same place checked the fuel pressure, re-checked the timing belt, tried a new Mass Airflow Censor, and eliminated other possiblities to narrow down to the CAT converter. The mechanic used a gauge to measure the back pressure but was not trained in how to read the results. At this point that shop was now guessing it was the CAT, and simultaneously I was getting impatient, and feeling helpless that nobody can figure out what is wrong. Now, the vehicle is at it’s 3rd mechanic, and after running another compression check, the mechanics impression is that that is the problem (though Toyota determined it was “within range”), and my vehicle still running/idling rough and sluggish off the line and at highway speeds. By the way, my CAT is an after-market (due to the original part being stollen), and I am told it is smaller than the original. I am told the backpressure is a little less at highway speeds, but should minimally affect the Hp at highway speeds. My mechanic now recomments replacing the engine with a 110K mile used motor. I am lost on what to do considering the situation and all of what I see, and hear, as guesses. Help!
It would help if you could get the exact compression numbers on each cylinder from your mechanic and post them here. But if the compression numbers were “in range” when measured at the dealer, I don’t see how that could be the problem. It sounds like your mechanic doesn’t really know what’s wrong and is grasping at straws.
You mentioned replacing the spark plugs. Have you tried new spark plug wires too? When was the last time you replaced those? What about other basic maintenance like new fuel filter, and possibly running some injector cleaner through the system?
I also think that you should actually report the compression numbers. “In range” doesn’t really mean anything, especially because a major question is not “how high” is the compression, but how well balanced is the compression between all cylinders. A long time ago before I knew anything about cars, I asked a shop to check compression in a 4 cylinder car I had. I had 2 cylinders down around 125psi and 2 others up at around 180. They told me it was “fine” - guess what? That’s not fine. I would have called fine if all were down near 125 OR all were up near 180. Anyway, report the numbers.
But more than a compression check you should actually ask for a leakdown test, especially because of the initial timing belt issue. I don’t know if this is an interference engine or not but damage to valves can occur either way.
Check for a Cat clog is not mysterious, btw. I’m quite surprised that you say a shop tested but had no one who could read the results. Odd. This can be checked pretty easily with a vacuum gauge and reading what is happening is fairly unambiguous. A quickie shadetree method is to temporarily remove the O2 sensors and see if it makes a difference. If there is back pressure buildup this just gives it a place to go. You would NOT do this for more than a short test drive. It’s a quick thing for testing only and will set codes and mess up how the car runs.
@cigroller, agreed about the compression numbers that “in range” is necessary but not sufficient. I assumed that the dealership would know how to assess compression numbers properly. But maybe not.
BTW, this is apparently not an interference engine.
@jesmed, if I know a dealership provides good and trustworthy service, then I trust something like that. But many do not. I’ve seen as many crazy reports coming out of dealerships as I have out of “national auto care” chain. It’s also the case that a compression test only measures peak compression during cranking. Leak down testing might reveal other things.
I run fuel system cleaner in my vehicle at least a couple times of year. I thought, initially, that that was the issue, but also remembering that the vehicle ran flawlessly before it jumped time. I will be back with those compression numbers as soon as I can.
@cigroller, I didn’t know jumped timing belt could damage valves in a non-interference engine. How does that happen?
If so, that could well be the problem since Pake says the engine ran fine before the jumped belt.
@jesmed, Consider that others here will be more knowledgeable than I, and I stand to be corrected.
Normally a noninterference engine will be fine with a timing belt problem. But non-interference does refer to piston/valve interference. I’m pretty sure this engine is a dual cam, and with a dual cam, it’s possible for the valves to interfere with each other if they are out of time.
Other than that I’ve just seen reports of valve damage despite even a single cam with non-interference design, though this would generally apply if the belt broke. This one apparently didn’t completely break. To me, timing problem followed by poor running always means - if timing is verified - to check and double-check for engine damage.
Interference engine means valve damage is probable - but not certain. Non-interference means improbable - but not impossible.
@cigroller, I’m just in learning mode here, since you know more than I do. So I’m just pondering/puzzling through this one.
I have seen reports of valves being bent in non-interference engines when the belt broke. Presumably from the inertia of sudden valve stoppage.
But in this case of jumped timing, not belt breakage, that shouldn’t happen since the valves aren’t suddenly stressed. And since all the valves in one head are controlled by the same camshaft, no matter what the belt does, there’s no way for the valves to get out of synch and hit each other, or is there?
Or is it possible that driving for a while with the jumped belt burned a valve or valve seat?
Not disagreeing with you, just musing aloud and trying to learn something.
I have the compression numbers. The passenger side, all 3 cylinders are at 150. Driver side from front of the motor to back are 195, 185, & 200
@jesmed, I don’t think valve damage is likely. But its not impossible (actually never is regardless of a past timing issue), so I think that before chasing one’s tail all over the car (as seems to be going on here) should never happen until one rules out mechanical damage. This car has been to 3 shops with all sorts of poking and prodding. Compression and leakdown tests are not wildly difficult or expensive to do, so I figure step one is to make sure the ghost isn’t hiding out inside of the engine. It’s the same logic I use when the computer won’t turn on. First, check that it’s plugged in.
@Pake, others with more knowledge than I will comment, but while the compression in each cylinder may be high enough (“within range”), I’d call those results a problem. Your lowest (150) is 25% lower than your highest (200), and the two banks are quite unbalanced. I’d want to know why the pass side bank is consistently lower than the other side. 150 across the board looks good all by itself. 185/195/200 is also ok all by itself. Next to each other on the same engine - not so great. I’m still saying I’d want a leakdown test.
Compression is unbalanced, yes, maybe due to slightly different oil pressure/temperature/flow conditions in the two heads causing uneven wear over time? Or maybe slightly different coolant temps. But are those compression differences enough to cause sluggish/rough running? I’m skeptical, especially since the engine was running fine before the belt skipped, and the 3 cylinders at 150 psi didn’t suddenly get there because of that. More likely normal wear on an older engine.
Not saying don’t investigate compression further. But at first look I don’t see a smoking gun there, and I certainly wouldn’t swap an engine out on that basis.
I’m wondering if there’s something more subtle like a vacuum leak. You could try spraying carb cleaner around the intake manifold at idle and see if it smooths the idle out (careful, don’t set it on fire). Also have your mechanic check manifold pressure with a scan tool.
And like Cig said, your mechanic can temporarily remove the O2 sensors for a test drive. If that helps, suspect plugged cat.
Thank you both for your advice. Sounds like a leakdown test is the next best option, along with pulling the o2 sensors to check the CAT. After work today I’m driving my vehicle with the mechanic to further go over the issues. Hopefully he will better understand what I’ve been talking about and experiencing. I’ll probably be back again next week after I have him perform those tests, for futher knowledge and advice. Thank you again!
Assuming all the obvious stuff has already been eliminated, ignition & valve timing, vacuum leaks, egr, pending codes, etc. The cat can be ruled in or out by simply temporarily disconnecting it and seeing the problem goes away. The imbalance in compression between the two sides is sort of suspicious, but I concur w/the above comments, seems unlikely that alone would cause a noticeable performance problem. One thing you could do is to go to a shop which has the Toyota scan tool ( and a tech with training on how to use it) and ask them to do a real time analysis of the engine performance, including O2 readings from both banks and the fuel trim parameters. That might turn up something.
Those compression numbers bother me. You say that your truck ran flawlessly before the timing belt issue and now runs poorly. The right side compression is all significantly lower than the left. I would want to double and triple check the camshaft timing at that point, perhaps even going as far as making sure the cam sprocket hasn’t slipped on the camshaft on that head. Compression that is that far off one side to the other seems to me to point to a mechanical issue related to camshaft timing.
Exhaust backpressure isn’t measured with a vacuum gauge, but with a backpressure gauge in the exhaust. It’s a simple tool that any decent shop will have. But since your truck has a single exhaust system, it’s unlikely that a plugged exhaust would cause compression issues on one side of the engine.
Without more live scan data and perhaps a manifold vacuum waveform from a labscope, I’d be pulling timing covers and sprockets first.
@asemaster, that’s an interesting theory about the cam sprocket maybe slipping. I could be wrong but it looks like the cam sprocket is keyed to the camshaft on this engine (5VZ), so slippage may not be possible. Worth checking though.
The compression differences are a huge issue that needs to be sorted out before anything else.
It’s either a mechanical fault due to rings/valves or the possibiity that someone is off a tooth or two on the camshaft timing on that bank.
Given the consistency of the readings and the statement that things were fine before, one has to suspect the latter.
If someone is performing the task of a compression check it seems to me that would be the time to sort out the problem.
Those keys can and do break, and this is especially prone to happen if there have been belt problems.
On the issue of a vacuum gauge, I was not suggesting that one use it to measure exhaust back pressure. Of course it doesn’t measure that. It’s a vacuum gauge so it measures vacuum. A vacuum gauge is a very simple device that is simpler to use and typically cheaper than a back pressure gauge. As such, it’s more likely the kind of thing that one might be able to locate among friends. (I have one). Obviously the shop with an actual back pressure gauge was of no help whatsoever. A vac gauge will not give a direct check of exhaust back pressure, but it is an easy way to find out if exhaust restriction should be a suspect or not by simply getting a baseline vacuum reading, and revving the engine. If under steady rev the vac remains stable (and probably rises an inch or two), then the exhaust is clear. If it drops off while revving, assume exhaust restriction. It takes about 2 minutes and does not involve tapping into the exhaust pipe.
My vacuum gauge shows vacuum on one side, and pressure on the other side
My gauge set is a lot older than the one in the picture, but it can measure pressure, to a point