DupermanDave

I have various comments and threads on this forum about my issue. My truck has been running rough at idle for a few months now.

Right now, it idles rough at stop lights for about 20 seconds. It will sound like a motorcycle. It usually clears up and sounds a little better after 20 seconds, but within that 20 seconds the RPMS rise and drop by about 100 rpms. It's just a twitch, but the sound is noticeable.

It started, coincidentally, the same time I took out my air filter to clean it (as well as the EGR modulator) I thought it could be too much oil from the K&N filter on a sensor, so I removed the VAFM and cleaned it thoroughly. Still nothing.

Since it only happens when the engine is fully warmed up I decided to replace the coolant temperature sensor (the one by the cold start injector). No change there.

I also had the idle surge issue a while back. The one where if the brakes are pressed, the idle RPMS go up and down and up and down. I fixed that by adjusting the TPS. TPS and idle screw are perfect. It normally idles at 800 rpms now.

I was told it might be the cold start injector, so I unplugged that and got no change. Cold starts started just fine, almost like the injector was never unplugged. I saw no change in the way it idles.

I just timed the engine yesterday and saw a slight change for the worse. My engine now hesitates upon heavy acceleration and still stumbles and stutters at idle. I sort of fixed the hesitation by unplugging both O2 sensors (i have the California model. 1 sensor before and 1 after the cat.) With the sensors unplugged it no longer hesitates, but it still runs rough.

Accelerating is not an issue. No misfires or backfires. It afterfires very occasionally when I hit 3k RPMS and I decelerate.

So before I remove the whole engine and replace it with a chevy 350...I need to get some pointers on where to look. I don't want to replace the engine piece by piece. I've googled and searched forums and found no solution. Most forums have people with the same symptoms, and in the end there's no answer in the thread. It gets annoying.

So where should I look next? I think replacing the oxygen sensors should be the next no-brainer, but that's where I draw the line. After the sensors I don't want to replace parts unless there's absolute reason to do so.

904_runner

Check the cat, iit may be clogged.

You say it ran better withought the o2 sensors, which may be letting the exhaust gasses get past teh cat which might make it seem better..

Try unbolting the front of the cat and see if it helps.

My $.02

DupermanDave

I left the O2 sensors in. I just unplugged the connectors.

904_runner

oh... hum..

Check the tps as well..
When you adjusted teh timing did you have the jumpers connected?

DupermanDave

Yup. Adjusted TPS and jumped the diagnostic connectors to time the engine. Everything was done as per the manual's instructions.

I edited my first post to reflect the TPS adjustment. It might be useful for those first reading this.

904_runner

I just reread it, i would change out the o2 sensors since unplugging them made it run a little better, but still rough.

It may have run a little better since it wasn't getting inaccurate signals from the faulty o2 sensors (if that is the case) but still run rough since there was no o2 sensors connected.

Im not sure of the problem just trying to help you narrow things down.

DupermanDave

I'm working on removing the sensors now. I want to get the nuts free and then I'll order the new sensors. Once the nuts are free and I have the sensors I'm going to take a sunday afternoon to replace everything. I just dont want to get the sensors and then not be able to remove the nuts, so I'm progressively working on getting the nuts off.

I'm also stalling so I can save up money to order the sensors. Haven't been working much lately, but work should pick up next week.

DupermanDave

My truck is also hesitating a lot when accelerating. The RPMs go up, but the speed increase is minimal. Almost like my clutch is slipping, but it's not. I know it's not because I can mess with the distributor and the hesitation will go away.

Will the o2 sensors affect this? Even though the hesitation seems to be determined by the distributor's position and timing?

Flash319

I would make sure your intake tube is not damaged. A very slight crack or any leak will cause rough running. If you have another tube try that. I have had one where I could not find anything wrong with it but it must have been leaking because I swaped it with another and the truck ran like new.

DupermanDave

I swapped in my old intake tubing and had no change. I have a new spectre intake tubing installed, but I did put it back to stock this evening before going to dinner.

I'm going to be ordering the o2 sensor(s) friday. I have a quick question. I have the Cal. model, and it has 2 sensors. Does the computer need both sensors to run properly, or can I only replace one?

sb5walker

The computer probably only uses the front sensor to adjust mixture - the back one is to verify the cat is working - like the EGR Gas Temp Sensor, verifying for the edification of the California Air Resources Board that your emissions equipment is functioning properly.

You said your idle problems came in when you cleaned your egr modulator, and it's possible something got messed up within the modulator or possibly the two vacuum hoses from the throttle body got reversed. Vacuum passes through the modulator on the way to the egr valve: it is not supposed to open the egr when the motor's at idle, but if the egr DID operate at idle, it would definitely make the motor run rough. The fact that it only happens when the motor is hot is consistent with this, since the vsv prevents the egr from getting any vacuum when the motor is cold.

Try pulling the vacuum hose off the egr valve and plugging it. If you no longer have the rough idle, verify the two hoses from the throttle body are on the right ports on the modulator. If you can't tell, you can always try reversing them. If you can't get the egr to work and you have verified that all the vacuum hoses to the modulator, vsv & egr valve are routed correctly, you probably need a new modulator.

DupermanDave

Ok, so it's completely safe if I just replace the front O2 sensor and not bother with the rear?

Also, I didn't really clean the EGR modulator, I just removed it and opened it up to check it out. It is a possibility I reversed the hoses, but unlikely. I'll mess with those hoses today and take it for a test drive.

I should point out that I figured it could be a vacuum leak, so I replaced all 3 hoses going to the modulator.

sb5walker

It's safe, but if both sensors are reading too lean (usually how they read when they get coated with carbon), fixing the front may reveal to the computer that the rear is bad. The computer can detect that the rear sensor is bad if, while at full throttle for two seconds, the front sensor detects rich (as it should) and the rear sensor detects lean (as it should not). In that case the computer will throw a code 27. It will also throw 27 if the rear sensor's heater circuit is open or shorted. Strangely, there appears to be no code to identify a bad cat, which one would think was the whole point of putting a sensor after the cat. If someone knows otherwise, please post.

So it's safe to wait for the code 27 to tell you that you need to replace the rear sensor - the mixture adjustment is done based on signals from the front sensor.

Good idea to verify routing of the hoses. Here's a diagram for a 93 - I checked my 89 fsm & the routing is identical:

http://www.ncttora.com/fsm/1993/engine/15layou.pdf

Also not a bad idea to learn if the egr is causing the rough idle. If the idle clears up after pulling the top vacuum hose off the egr valve and plugging the hose, then the egr is opening at idle, which it shouldn't be.

Another thing worth checking is the signal rotor clearances (on all four lobes) and the signal coil resistance in your distributor - spark timing and advance is based on those signals.

DupermanDave

just checked the egr. All is fine with it. When driving the truck, however, it is running worse than ever. It almost stalled out on me on my way to the globetrotters game.

-EDIT-
WOW!!!! Tonight, on the way home the truck idled and drove perfectly. I have never heard it idle so perfectly. No stutter, hesitation, or stalling out. I'm clueless and getting more and more frustrated. I have changed nothing on the truck other than remove the egr modulator and reinstall it.

-additional edit-
Truck now backfires in second gear every time I shift up to third gear and hit 2500 RPMs. Again, I haven't changed anything. I'm going to leave the o2 sensors unplugged.