Excess vibration but not missing

redbowtie · June 18, 2002

Let me say in my first post that this is a really nice site you have.

I have my 90 Mark 3 van, and have been having a lot of problems with it lately. I have tried in many places to see if anyone has a clue whats going on, but no luck.

The motor has 148k on it.350 TBI. About 2 weeks ago it started to run rough. Not missing a cylinder rough, but like you were sitting on the exhaust manifold rough. It vibrates like crazy. None of the exhaust hits anywhere. It has also lost a lot of power, but will still drive.

Heres what I have done....

Replaced ignition coil, cap, rotor, wires, ign module,all plugs (which all looked normal, possibly a little white, but all were consistant)fuel filter, air filter, checked O2 sensor and is working properly. According to my scan tool, it warms up normally to about 195*, no major coolant temp fluctuations. Timing was checked, pulls plenty of vacuum(>20in at idle). Does not set the SES light for any codes. Plenty of bat power, good oil pressure, battery is new. Tried cleaning injectors with no change. Richened mix while driving, no change. Tried tank of 94 octane, with plenty of booster, no change.

I have a lot of experience and the tools necessary to fix most cars, but this is really whipping me. I am ready to dump it in a garage and let them deal with it. The problem is I have no faith in the local shops either. They will sell you things you dont need, repair things that dont need it, not fix the main problem, and charge through the nose for it all.

Anyone have anything else to try? I am really at a loss here. The only other thing I can think of to go after, is the cat converter is plugged. But, to get that monster in the air where it sits is nearly impossible.

Thanks in advance guys!!

redbowtie · June 18, 2002

I also should mention, since it is important...

I had to replace the d/side headgasket in January. I am not sure of the exact cause, but I believe it was overheating when the alternator seized and ripped apart the fan belt. However, it does not run like it did when it was blown, nor does it show signs of coolant in the oil.

Since I am not overly familiar with TBI, how do you check the fuel pressure? If I remember right, the regulator is sealed and in the TBI unit. There are no provisions for a gauge anywhere like the TPI and MPFI I am used to.

It is possible that the pressure is low, the plugs look like it has been running lean. How do I check this? :ehh:

And what is within spec for this motor?

June 19, 2002

Does it run rough only under load, or at idle/in Park/Neutral as well?

I'm just wondering if it's not the tranny, driveshaft, or rear end that could be the culprit.

redbowtie · June 19, 2002

It does "vibrate" slightly at idle. Enough to notice that something isnt quite right. I mean, even at 148k, the motor was smooth as butter up until now. At a hot idle, it wants to stall out, it doesn't, but it kinda "hiccups" and recovers.

I thought about the tranny too, but it was actually just rebuilt a few months ago.

It does seem to happen worse under load however. If I am driving at 55, it vibrates like crazy. If I floor it, so it shifts into 3rd, (700r4 trans) it clears up a little for a second, then goes into it again. Although, I think its just the tq converter locking up and amplifying the problem.

As soon as I let off the gas, it hangs there for a split second, then the vibration reduces back down to idle level. This leads me to believe its not trans or driveshaft related. I also checked the d/s for any buildup causing it, and nothing there either. When going downhill though, and having the engine doing some of the braking load, it roughens up a little. Im pretty certain it is load related, but it starts in even feathering the gas on level road.

One more thing to add. I am leaning (pun intended) to say its short on fuel. The plugs look abnormally white, evenly all cylinders. Also, while I had my scan tool hooked up, it threw a lean exhaust condition code. However, it did not trigger an SES light. At times, I watched it, warmed up, the ecm would go from closed loop into open loop, and the blms would jump to 172. It would run like that if I left it alone. If I got on the gas and brought it off idle, it would go back into closed loop and the blms would stabilize at about 125-130. As far as I can tell, the O2 is good, gets very active once its warmed up.

If there is a way to check fuel pressure, I might be able to determine if the pump is on its way out. But how?

95 Silverado · June 20, 2002

I have heard of instances where the torque convertor clutch can cause some strange performance problems. My father thought his transmission was going out, it would buck and shift irregularly on hills or under a load, turned out to be a bad plug wire that really acted up when the torque convertor locked up, this puts a greater load on the engine. I've also seen convertor controls cause the convertor to "hang up" or "shudder" making it feel like an engine or internal transmission problem, many times the switches and solenoid that control the convertor are reused during a rebuild. It may not be your problem but it might pay to look into it.

mnstrmdnss · June 20, 2002

im not really familiar with tbi injection, but i do believe that you need to check fuel pressure at the fuel tank. im not sure how,, but according to the tech at my work, thats were you check it. ill talk to him today and find out exactly how.

rrsob · June 20, 2002

I'm not a mechanic by no means, and this may be a stupid question. But,could this be caused by a broken motor mount.

I had a car break one once, and it vibrated like mad. Rich ??? ???

redbowtie · June 20, 2002

Tranny's not the problem. I took the van back to the trans shop to see, and he checked it out. He couldnt find anything wrong with the tranny. If the TCC was the problem, I could take it out of the equation by stepping on the brake while still on the gas. It unlocks the TCC at speed. I tried that, and hardly any change.

Motor mounts... if it was, I think that it would be just as bad if I let off the gas. The motor is still turning anyway. Besides, I looked them over and they look really good. On a van, you can look right at them while its running since you're sitting on the motor practically.

I think I might call one of the local places today to have them drop the tank and check the pump out. Im really not confident enough in my skills to play around with a gas tank. Especially on something this old, the tank really may not want to come out without a fight. I just hope I can make it to the shop, and back if need be. Its a 20 min drive either way

One thing I tried last night.... Started it up, and let it warm up. Turned it off, and sprayed starter fluid into the TBI. Started it back up and it seemed to run a little better for a few seconds. Hmmmm.... :ehh:

Shaners · June 20, 2002

I wonder if the harmonic balancer is bad...

Colorado Kid · June 21, 2002

When going downhill though, and having the engine doing some of the braking load, it roughens up a little.

If this is true it is a big flag to tell you you're barking up the wrong tree, because (and your scan tool will verify this) the TBI shuts down completely...zero fuel delivery...after about three seconds with the throttle closed and the rpm above 1200. Your problem is not in the fuel or ignition systems.

A plugged cat is very smooth, but very gutless...also not what you've got.

Does it shake if you wind it up to cruising RPM in nuetral?

The harmonic balancer and the torque converter would be my prime suspects for a sudden onset, engine speed vibration. The low power and erratic idle make be caused by the vibration messing up the knock sensor resulting in intermittantly retarded timing.

It could even be one of the belt driven accessories failing, taking a lot of power to drive and rumbling. (A/C compressor?)

Vibrations and noises are hard to diagnose when you're there, even harder second hand. I hope this helps.

On Edit: Oh yeah, if it doesn't shake on overrun (coasting down hill) then I have one more idea. I have seen one of the two injectors on a TBI fail, either intermittantly or completely. This pretty much kills one bank of cylinders and screws up the mixture some on the other one. This causes chugging at low speeds and vibration at higher speeds along with a huge power loss. When intermittant the power will sometimes come back on suddenly. The test is to disconnect the electrical connection to one injector at a time while its running...one will make no difference, the other will shut it right off.

redbowtie · June 21, 2002

Well, I thank you guys for your input.. but this is what the shop had to say when I brought it in today.

Blow by. Piston rings are shot, pistons are damaged. Needs a new entire motor.

C'mon, this is BS aint it?? The damn thing doesnt really use much oil, the cylinders tested fine compression-wise after doing the headgasket change. The plugs look evenly white.They arent oil-soaked in the least..!! And how in heck are you gonna find that out without pulling the heads off (which they didnt do)?

Heck, my dads 1991 Buick Roadmaster has 175k on the TBI 305. Blows the dipstcik right out and uses about a quart of oil every 2 days. Smokes like a wildfire. Runs fine. Little loss of power, but no major running problems. Same thing with the 307 I had in my Regal. Used a quart a day and blew it everywhere. Ran good.

Frankly I think Im getting screwed with, but honestly I have come to expect that of the shops around here. But, this was a local well known Chevy dealer. Still I think its total BS. Am I wrong to doubt their finding? They had the thing all day today while I was at work. The service mgr went home by the time I got it back, so I really dont know other than what he told me over the phone.

I think I need a second opinion. :angry:

Colorado Kid · June 24, 2002

I agree, the shot rings/damaged pistons theory is nuts. One easy check is just start the engine with the air cleaner lid off, put your thumb or finger tip on the top of the breather pipe. If it gets sucked onto the breather the blowby is less than the ca[acity of the PCV system (not much at all). If you can't hold your finger there because the air is wistling out of it, (and covering everything with oil) then you have a blowby problem.

With a truck this old it's time to meet a good independent mechanic. If you have a favorite parts house (Car Quest, Napa, Parts Plus) ask them to recommend one. I've used the trick of calling the local NAPA and asking for a recommended mechanic to find good help a long way from home a couple times.

redbowtie · June 27, 2002

Anyone else have some thoughts?

My gut tells me its the pump (or strainer) and that should be replaced. The place I took it to didnt even bother with it.

Im thinking of taking it to someone and tell them to just replace the pump & strainer and nothing else. I kinda hate to go out on a limb and spend that money on it, but I might just have to. I wish I knew how to check the fuel pressure. My turbo regal is easy, I have a gauge on the end of the rail on the schrader tap. But where the heck can you check the vans pressure?