I first noticed a laggin' going uphill and then I noticed a skip. It starts good and idles fine for the first couple of minutes and then you can detect a skip at idle. While driving the skip is obvious as it feels like something is choking the engine intermittantly(sp).

I've pulled the plugs and all is fine....wires and plugs. I know it could be timing and all that crap but it's got a belt so it wouldn't just slip a notch right? And as far as the idle speed is concerned (thought about that too) that wouldn't slip suddenly by itself right? It's gotta be something else bigger that I'm overlooking.

For those of you who are familiar with my truck, remember that I just rebuilt the engine (sort of, I used most of the old parts but did strip it down to the block but replaced the head gasket).

There are no drip, leaks, or any other such problem. The engine turns, starts, and runs...it's just NOT HAPPY.

Help!!! Any suggestions would be more than welcome.

I will mention that there is not a usual exhaust. I took out the cat (a long time ago) and put a straight pipe for the time being). The exhaust is piece-meal...lots of clamps and ***** but brand new muffler). The only reason I mention this is cuz it seems the trouble started after we messed with the exhaust last week to put the new muffler in (yes, we put it in right). But it just doesn't feel right that it could be the problem.

I have a similar problem on mine that developed after little welding got done to it, so if you did any with your new muffler going in and all that it may be related to your electronics. Otherwise the sky is the limit, trouble shooting of intermittent problems is a bit tricky. If you think it is related to your ECM check out my post which is just next to yours on Trooper ECMs.

You gotta remember an engine is a system. You change one thing and it affects everything else. There's a certain calculation of back pressure that is designed in the exhaust system that pushes back against the pistons forcing the exhaust gases out. Without that back pressure there's an undue strain on the exhaust valves which take a beating anyway from the hot gas. If you're losing power and it's not something obvious like your gas filter was clogged, you have to go back to where
you started when you put it back together. Make sure your timing belt is in the right position with the timing mark lined up and the camshaft mark in the right position relative to the timing mark. If they are not dead on you will never get the timing right so take the timing belt off, line up the marks and put it back together. Before you start your engine, make sure your distributor is lined up correctly relative to the timing marks. Mark the #1 socket on the cap and the side of the distributor. Pop the cap. If the rotator is not pointing to the mark on the side of the distributor you need to pull it out and move it until the gear at the end of the distributor meshes with the gear on the end of the cam with the rotor pointing to the right place. If they check out, get out your timing light and set the timing. If it's still not running with power do a compression check. I hate to say it but loss of power is often an indication of valve problems. You may want to pull the valve cover and make sure the valve settings are correct (cold engine). Valves set too tight or too loose are trouble. Don't ignore the symptoms and keep driving. It's a PITA, but the alternative is turning what might now be a simple problem,
into another rebuild. Been there, done that.

Yep a valve dropping into a piston is a pain in the <img src="/forums/images/graemlins/butwiggle.gif" alt="" />. BIG mess lotsa parts that don't work anymore, foul language and neighbors offended by the language <img src="/forums/images/graemlins/shiner.gif" alt="" />

Thanks so far guys but it passed a compression, leak-down, and vacuum test. I know about the exhaust thing but didn't realize how much strain I was putting on the valves by removing the Cat.

Though I mentioned the engine was rebuilt I don't want you guys to be mislead.....This was done in May/June and the truck has run beautifully since. This problem seemed to develope just after installing the new muffler.

As far as the exhaust is concerened...from the head pipe to the flex pipe, all is original. Then there is a straight pipe in place of the cat, then there is the new muffler and original tail pipe.

I left it at school where I have a warm place to work, plenty of tools/equipment at my fingertips, and the expertise of the instructors and seniors readily available.

I am still wide open for suggestions. The instructors have mentioned valves as a possibility but they are also thinking that it is nothing too serious and may just take some time to diagnose. At least I get lab credit for working on it but I'd much rather be driving it!!! <img src="/forums/images/graemlins/smile.gif" alt="" />

Dawn, I think you're confusing cam timing with ignition timing. Both will make your engine run poorly if they're off, but they are two separate issues.


With the crank and cam marks lined up, you're at TDC of #4's compression stroke, so point the dizzy to #4.

<img src="/forums/images/graemlins/cheers.gif" alt="" />
Phil

Thanks phil for the input but unless the belt, the dizzy, the cam, or some other mysterious part come somehow just jump a notch then there should be no reason why it would suddenly be outa time.

So far we have done the compression, leakdown, and vacuum tests as mentioned above as well as a heat check of all four cylinders at the exhaust manifold with a lazer pointer.

Yesterday we decided to check and adjust the valves cuz a dollar bill placed at the exhaust was sucked in instead of blown out. This happened only one time but it may have indicated a sticky valve. While it didn't solve the problem, we did discover that a few of the valves were out of adjustment.

Though I have little experience, I did get intimate with this engine as I've seen it torn down to the bare block and reassembled it to perfect working condition. My troubles began after installing a new muffler. And my instinct keeps bringing me back to the ECM. I mean think about it. There is a straight pipe where the CAT is supposed to be and I'm told there needs to be a certain amount of back pressure with that system. The ECM is operating with the idea that there is a CAT installed.

Let me explain what I'm thinking and you guys can tell me whether I may be on the right track or way out in left field somewhere.

I've discovered that the truck runs without skip on cold start for about 3 minutes. Once it reaches the point where it kicks itself down, the skip becomes evident and pronounced. My thought was that the exhaust is sending mixed signals to the ECM and the ECM is sending the wrong signal to the gadgets (all those darn sensors) that adjust the air-fuel ratio. Then upon further scrutiny, I discovered the open-loop...closed-loop function of the ECM. When the engine is cold, it is running in open-loop cuz there is not enough info with regards to the air sensors to be sent to the ECM. Those sensors need to be up to a certain temp before they report. Once they are warm, the loop then becomes closed (the sensors send their information) and the ECM makes the adjustments according to the data it recieves.

Since my truck runs fine durring cold start and for the first few minutes, I'm leaning towards a bad sensor somewhere in the group that does not function until warm.

Whaddya think? Could I be on to something here? We plan to use another specialized piece of equipment today to test the air-fuel mix being sent to the injectors today. Maybe I will get some good info.

I have to say, though I hate leaving my truck somewhere other than home, I cannot think of a better place to be than at the tech. where I have two really great teachers with a whole lot of patience, willingness to teach, and the tools...not to mention the heat. These guys know I could just send my truck to the garage to be fixed (one instructor owns his own family run garage) but they have implored me to use my truck as a learning experience. I wonder if they've got more confidence in my abilities than I do.

Today we hooked the truck up to a scan tool. Not sure what exactly it was called (just know it costs about $5000.00)but it measured air-fuel ratio as well as many other things including NO2. Based on those readings (one number was up around 1300)we decided to disconnect the O2 sensor and the truck ran great while at idle. That number (wish I could remember what it was) dropped to between 350-450. So we thought we had the problem solved....buy another O2 sensor....wrong. Test drive brought it right back to where it was.

Next step, run the fuel pressure regulator with injector cleaner. Once again, the truck ran a bit better for a mile or two and then the symptoms returned.

I then tested the fuel pressure regulator by disconnecting the return line to the tank after the regulator. I started the engine with the pressure guage on to see what would happen if the fuel was routed through the regulator into a bottle instead of to the tank. The pressure went up to 48 pounds with the line routed to the bottle but dropped back down to 40 with the line routed to the tank.

Fuel pressure regulator??? Other suggestions???

Help....please....

How close is the O2 sensor on the 4 cyl trucks to your former catalytic converter location? On my V6, it's very close.

Since the cats are noted for the heat they generate, could be the sensor is operating much colder than intended and sending a strange signal to the ECM.

When operating w/o the sensor, it was maybe using a default value that went back to the 'strange' value with the new sensor - IE, the original sensor is/was working, just sending a screwy signal (as far as the ECM is concerned).

Just a thought...I'll be interested in seeing what you finally find out, mine has been doing the same thing since I got it, but I haven't had the time or resources to do the level of investigation you have going, I've just been living with it!

Jay

K, first of all, the test was the Exhaust Gas Analyser and it was the O-2 reading that was off the wall with the O-2 sensor connected and dropped significantly while disconnected.

Jay, I'm sorry to hear you are experiencing the same symptoms. Even my auto instructors are perplexed but they keep plugging at it. And these guys are the best.

As far as the relationship between the O-2 sensor and the CAT are concerned....on the four cylinder engines there is one O-2 sensor located on the head pipe while the cat is located just after my flex pipe which is relatively half way between the head pipe and exhaust pipe. I know that some engines....(mostly late model or larger engines I think) have 2 O-2 sensors...located just before and just after the cat.

You do hit on something when you mention the "default value" that the ECM will revert to. This value is referred to as the "limp-in" mode which basically sets the ECM to basic values so the truck will run without the O-2 sensor reading in order for you to 'limp-in' to a service station.

There is also something known as the "open-loop" and "closed-loop". On cold start the truck is running in "open-loop" meaning that basic values are set because certain sensors have not reached operating temp and cannot yet send their signal to the ECM, and the air-fuel mix is rich. Once the sensors have reached operating temp the sensors send their signal and the ECM adjusts the air-fuel ratio accordingly. I had thought this might be the problem because the truck runs great on cold start and then once it warms begins to run like crap but it does this with or without the O-2 sensor connected.

Anyways, thanks for writing in Jay and I will keep this thread posted in my progress. Today, it's back to the drawing board. My instructors won't let me give up and send it out...there's really no need I guess. One of them owns a service station but look at the money I am saving do this on my own again, and I get experience and lab credit too!