94 3.8 freeway hesitation

Recently acquired 94 3.8 TS hesitating above 40mph. SES light was on, and I took it to a dealership to the tune of $900 (they replaced a computer - the ECG? can't find the receipt). Now light is off, but hesitation is worse, and the dealership went out of business. Today I changed the fuel and air filters and put some injection cleaner in the tank, and it seems to have more power in general, but it's still hesitating (we'll see if it gets above the 17mpg it's done so far).

It happens most often during gradual acceleration, somewhere between 40 and 70mph, feeling as though there is a headwind or perhaps as though the transmission is shifting up and down repeatedly, but I don't hear any change in the engine noise (once or twice I thought I saw the tac dip in time with the resistance, but usually there is no visible change). If I'm travelling at constant speed, sometimes it will go through hesitation cycles, where I lose power for one second and its back the next - on/off/on/off/on/off. The electrical seems fine. If I stomp on the gas while it is hesitating, it accelerates normally (I think normally - I haven't had the van running well enough to know for sure what is normal with it).

Another problem that may be separate or related is that it starts to shudder around 70mph (which, in Sacramento, means I'm being passed on all sides as I baby my heap).:sunglasse

Looking at forum posts and my pocketbook, I'm figuring on checking the ignition wiring (although that will be a challenge for me, and I don't have an ohm-meter, so it will pretty much be a tug-a-little and tighten-a-little job), cleaning and checking wiring on the MAF. It seems like it is more likely to be the TPS (which shouldn't be too painful to change), but then again some of the posts on here sound like the TCC might be part of the problem. I'm not real sure what that is or does - when is it "applied" and how does one tell? Then again, it could well be the sparkplugs which, given my limited tools and facilities, would mean another trip to the shop.

Anyway, since none of the other problems on here sounded exactly like mine, I thought it was worth posting and am wondering if any of what I mentioned above can be eliminated as the problem so I'm not wasting time I don't need to; also, are there any other possibilities I should take into account?

Quite a helpful forum. Glad to have happened upon it. :)

I had a similar problem that got worse. It was a bad coil.

Check the coils to see if they are up to spec.

INdeed "hesitation" is indeed a feeling that can be associated to each of the causes you mention and you have to go by elimination.
1st, you can either check (see adjacent thread) or change TPS...but a TPS code inhibits 4th and TCC...so you would know if this happens...4th and TCC gives near 1800 rpm at 60mph; 3rd and no TCC near 3000 rpm.
2nd: ignition wires: last summers, my .93 really behave as if TCC was "shuddering" dramatically, and getting worse and worse: my ignition wires were beautiful, clean, ....and original equipment: but they were shot: changed them and that is a moment where I could have believed in miracles. If your ignition wires are OE, change them....I did not try to route the new ones through the front original path: just got around the dog bone.
Yes clean the MAF with alcohol, www.avigex.ca/xport/maf4864s.jpg at least to eliminate this possilility for sure.
It is always possible to disconnect D wire from the TCC plug www.avigex.ca/xport/tccconnector.jpg but disconnecting TCC can simply smooth out the problem while the cause is still somewhere else.
www.avigex.ca/xport/tccshudder.jpg

Thanks for replying so quickly. My rpms at 60 are 2000 or less, so that mean the TCC is working and eliminates both that and the TPS as potential problems, right?

Cleaning the MAF is free, so that's topping the list. You said above, LMP, that you took out the throttle body to clean the MAF, and I found your diagrams for how the parts fit together, but it looks from the diagram like it may be accessible without disassembly. Am I wrong about this?

I'm curious why an ignition problem wouldn't also cause hesitation at low speeds.

Where can I find out what the coil specs should be, and how do I check them? (I'm new at this - what do the coils do?)

Ignition wires are about 40$ and at that age, it is normal maintenance anyway..even if it does not cure the problem but at least, it clears the question.
well TCC "could" slip by bits and the average rpm stay close to normal...and a D disconnect would definitely remove all shudder if it were the actual problem..but of curse you would then be around 2400 at 60 mpg, and loose a few miles per gallon at highway speeds..would not change mpg in city driving since TCC applies above 50 mph anyway. I consider TCC failure not worth all out repair: I'd run withiout TCC.
A worn out CV joint causes low frequency shudder....very difficult to identify positively until you change it though.

Well, cleaned the MAF but it didn't help. Not sure I can quite decipher the ignition module diagram to correlate it with what's under the hood.

I did notice something I had not before that seems to be a pattern. Accellerating up an onramp works great, but if the onramp slopes down from an overpass, I'm hesitating like crazy - I guess this would suggest the problem is more likely when the van is "not under load" ... in case that changes the equation at all.

At this vintage grounds can sometimes play havoc with electrical/ignitions circuits. The ingnition module is grounded on a stuf hidden under the module itslef, I had a broken wire there. Also there are several ground on the front of the transaxle by the negative batery connectiion, make sure all of them are clean and tight

I have a similar vehicle - 1994 3.8 TP, which has had the same roblem in the past. I solved the major problem with the THIRD replacement O2 sensor. The sensors were going open circuit intermittently, which was proven by data-streaming with the OBD scanner. The O2 sensor went "flatline" at .42 volts whenever the vehicle started to surge, buck, hesitate, or whatever you wanted to call it. The first 2 sensors were NAPA/Bosch. I finally put in an AC DELCO and the problem has been gone for almost 2 years (van has 361000km on it, about 90000 on the engine)
With low fuel on a washboard surface I still have another issue. It seems the fuel "dances" off the bottom of the plastic tank and the pump draws air. If I don't back out of the throttle the engine will stall - requiring me to release the air from the fuel rail by depressing the schrader valve before it will restart. I just try to keep at least 1/4 tank of fuel and the problem does not occur.

...The O2 sensor went "flatline" at .42 volts ...which was proven by data-streaming with the OBD scanner. .
INteresting....and can I suppose this would not even generate a CEL code ?...or did it ?
NOw since this is valid only in closed loop operation, I wonder if forcing the PCM to revert to open loop default values could inhibit the bucking and be used as a way to diagnose the possibility such a fault as this kind of hesitation is reported quite often ....
WHich data-screening system did you use? I got an OBD1 serial cable from ALDLcable.com last year but my computer refuses to see the connection....
..what was the fate of the first engine ?