Running VERY Rough After Head Gasket Job

[vcp]

Finished a head gasket, head pipe, valve seal job yesterday and seemed okay until I got over 40 mph on the highway, then it ran like it was going to vibrate itself to pieces. Some surging, chugging, an intermittent 'knock/clunk' that seems to be coming from the exhaust manifold side of the engine, low power uphill, might be running at higher rpm than previously.

Took it back to my mechanic today and he spent hours trying to figure it out. Seemed better until I got up to speed again, and then exact same problem. Wasn't running like this before all the work. In fact, it ran great at speed.

My mechanic is stumped and suggests you guys might have some ideas. Anybody?? Thanks!

PS I already gave him the following hints that I collected from searching these forums:
- bad PCV valve
- vacuum line pulled off of HAC
- accelerator pump
- bad ignition wire or spark plug (all replaced 6 mos ago)
- vac hoses
- intake and exhaust manifold bolts
- Set timing at 10 deg before TDC

[Petros]

double check the valve timing belt is correctly on the timing marks. Easy fix.

Verify that the distributor is indexed properly on the cam gear, and than check that the spark timing is correct, and the spark advance is working properly (requires a timing light, but easy to check using the service manual procedure). No spark advance or bad timing could cause that behavior. also easy fix.

consider only the things that were disturbed or changed during the head gasket change. Was the carb removed? If so double check all of the vac line routing, and all of the electrical connectors for the emission system. sometimes botched carb rebuilds can act this way if a check valve or internal part was installed incorrectly (or left out), but I would not suspect this unless the carb was opened up for some reason during the head gasket replacement, otherwise the carbs are pretty reliable and I would not suspect it.

It is also possible (though less likely) that something failed about the same time as the head gasket replacement: double check the choke is opening properly, and all of the emission control components are working properly (there are some simple tests in the service manual, it does not take too long to check them all using the manual procedures).

Another thing that could be coincidence is the fuel filter is partially plugged, allows low speed operation, but chokes off fuel supply at higher speeds/throttle positions. Also easy fix.

A weak coil, works at low speeds but shorts out at higher speeds, or higher engine loads. It would also be a coincidence, rare to fail suddenly, usually these failures come on slowly, but anything can happen with a 30 year old car.

Leaky fuel line ahead of the fuel pump, you will not see a fuel leak, instead it sucks air in rather than fuel, starving the engine of fuel at higher speeds. Hard to find, but easy fix. but at least inspect the fuel line carefully for cracks or damage from pump down to hard line before the fuel filter.

Good luck.

[vcp]

Wow, that's amazing, Petros! Thank you so much. I'll be sure to let you know what we discover.

[Petros]

I would guess it is something simple, like two vac lines reversed. You or your mechanic will likely slap your four head when he finds it.

BTW, it is easy to leave off a vac line or misroute it, use the vac routing diagram for your car and trace each of the vac lines from start to finish, also looking for cracks or loose connections on each one. This is nothing to be intimidated by, do one at a time and it be systematic about it, it will only take a bout ten min to check each line from start to finish. Sometimes the old vac lines can be very brittle, like a candy cane, and during the head R and R something got cracked open and is now a vac leak that leans out the fuel mixture at higher speeds. I have R and R more Tercel heads and carbs than I want to think about, and know the vac system very well, but I still have to double check everyone of them each time lest I inadvertently mixed several around to the wrong place, and it runs like crap and I think "oh no! what now?" I had one Tercel I rescued from going to the wrecking yard, ran like crap, the previous owner thought it needed a carb rebuild. I found nine vacuum leaks, and several misrounted vac lines. Ran good, no carb overhaul, once I sorted it out (that took most of a day to track them all down!).

[dlb]

my bet is on incorrect ignition timing.

[rer233]

My money's on cam timing.

[dlb]

i had to look up the definitions of ignition timing vs cam timing:

http://www.badasscars.com/index.cfm/pag ... prd370.htm

so incorrect cam timing means that the marks on the crank and cam pulleys would not line up, right?

[vcp]

Thanks for contributing, guys! Forwarded Petros' comments to my mechanic and he just replied as below. (This is a great bit of long-distance cooperative detective work, yeah? )

+++++++++++++

The timing belt is correct, the marks line up and the distributor can be adjusted to the recommended timing but then it runs worse, runs better when set too far advanced, but then I found that before as well.

There are two vacuum lines that go to the distributor that operate diaphragms that cause the timing to advance at certain times, I found that when the hoses were connected that the timing did not advance like it should and when I removed a line and plugged it off the engine idled a little better leading me to believe that the diaphragm was leaking and not allowing advance to happen as well as causing a vacuum leak.

So I think that is why the timing needs to be set so far advanced in order to get any sort of power. I have that hose disconnected and plugged right now.

What I can't understand is why the stumbling happens more now then before the head gasket.

The carburetor was not removed.

I didn't check that the choke was opening correctly so you may want to pull the top off the air filter and make sure that it is.

Oh and I don't think I hooked up that hose that takes heat from the exhaust manifold to the air cleaner, it's just for cold starting so wont affect the stumbling issue.

A fuel filter isn't likely the problem as it was working ok a couple days ago.

[xirdneh]

its rare but worth checking just the same
the two graphite spring loaded contact pins in the distributer cap
make sure they can be pressed down and that they come back out easily

[Petros]

I have seen and had it where the same thing with the distributor, have to advance it way too much to get it to run. something is not correct there. It might be off a tooth or two on the cam gear, that is an easy fix but sometimes take several tries to get it to line up. There is a procedure for correctly lining up the distributor rotor in the repair guides forum I think. Easy if you know the steps, can be frustrating by trial and error. I have messed that up a few times, and I have R and R'd Tercel distributor over a hundred times.

I did not see him say he checked the mechanical advance in the distributor, this is the more important of the two. the vac advance only operates at part throttle conditions such as hwy cruising, it improves the economy a little, it will not harm a thing to cap it off it the vacuum advance is leaking (cap off both vac lines, or all three if yours has three). It should run fine without it. but if the mechanical advance is stuck, the spark will not advance with the engine rpm and it will loose power at higher rpms. check with the dist cap off and see if you can manually advance the rotor and it should snap back, they get sticky and rusty. lube with penetrating oil like LPS, and wiggle it back and fourth until it is free. if that does not work it has to come apart and get cleaned and lubed (not a too big a job, usually the spray penetrating oil is all it needs).

The large heat riser tube will not affect operation, I have mine permanently disabled to always take cold air in, more dense cold air delivers a bit more power and economy. In icy conditions it is more important to prevent carb ice from forming, and it helps emissions some too. It is not just for cold start but to keep the intake air at the same temp all of the time, mostly for emissions control (keeps the fuel air mixture in a more limited range).

[dlb]

I did not see him say he checked the mechanical advance in the distributor, this is the more important of the two. the vac advance only operates at part throttle conditions such as hwy cruising, it improves the economy a little, it will not harm a thing to cap it off it the vacuum advance is leaking (cap off both vac lines, or all three if yours has three). It should run fine without it. but if the mechanical advance is stuck, the spark will not advance with the engine rpm and it will loose power at higher rpms.

i didn't think about this, very good point. i have had a few tercs that ran fine most of the time but intermittently ran lousy due to the mechanical advance being gummed up, causing it to stick in either position. remove the distributor cap and try twisting the rotor counter-clockwise. it should easily twist 1/4" or so at the tip, and should snap back to its starting position when released. if it does not, the distributor must be disassembled, the old gummy, dirty grease for the mechanical advance cleaned out, and new grease put in. it's not a bad job, takes a couple hours total if some parts are really stuck in there, less if things go well.

[Gottolovem]

My money is on the dissy being a off one tooth

[Petros]

yes, I agree. Every time I have found a Tercel that needs the spark advanced way off the mark to get to run it was always the gear indexed wrong on the cam shaft. But it usually still runs okay, so losing power at higher rpm indicates improper timing at the higher rpm. The advance being stuck to also make it behave that way, or being off a tooth (could be both too).

[vcp]

Thanks guys! Some brilliant stuff. Working to deadline on a project due tomorrow so haven't had a chance yet to get down and dirty with the car. Will keep you informed of ...hopefully ... progress!!

[vcp]

Sent my mechanic your comments and he replied as below. Does this help narrow it at all?? (I don't know if there are no vacuum leaks. And excuse my ignorance, is there a simple technique for checking each hose? Thanks!)

********
I didn't need to advance the ignition timing to get it to run, just to get it to have more power.

I lined up the marks on the crankshaft balancer to TDC and the cam pulley with it's mark, and I checked that those lined up before taking the head off in the first place. So can't see how the cam timing would be off.

If there are no vacuum leaks then my thought is the distributor not advancing properly but just don't know why it would act that way now but not before.