gmc

Hopefully someone can help; I'm about at my wits end.
93 4x4 manual
The truck has a miss at idle and at part throttle (1500-2000 rpm) and under light acceleration it stumbles pretty bad.
Some background on the problem. I noticed a few weeks ago when cruising at 45 or below in 5th gear I would get a slight stumble/hesitation when I tried to accelerate. I figured I may have just been lugging the engine and didn't pay it much attention. It starting happening a little more often so I added some heet and lucas fuel treatment to the tank thinking maybe I got a bad tank of gas. I didn't really notice any difference.


After looking over the truck, I noticed it still had the original fuel filter and plug wires (truck has just under 79k on it), some of the vacuum hoses were starting to crack and I figured it was probably about time for a tune up and some maintenance.

I replaced the fuel filter and the problem actually got a little worse.


Then I installed new NGK v-power plugs (old ones were gapped at almost .044) and it got a little worse again.


I then tested the TPS ( also tried disconnecting it while running- no change), VAFM and EGR and everything seems to be within spec. Timing is set at 12 degrees with the jumper, and vacuum at idle is a steady 18 inches.


This past weekend I replaced most of the vacuum lines, intake tubes, new distributor cap, rotor, plug wires, ECT sensor, fuel pressure regulator (all Toyota replacement parts) and swapped the injectors for a rebuilt set. And now it runs worse than ever.


It has a very noticeable miss at idle. If I hold the rpm at around 1500 or so it stumbles and misses pretty bad. Part throttle or mild acceleration when driving elicits the same response. Once the engine gets above 2000 rpms or under hard acceleration it does smooth out. I do not have a CEL but it has codes 24 and 31 stored.


I am thinking it is an ignition/electrical problem, but I can't seem to find anything wrong. I'm pretty much stumped at this point. Hopefully this all makes sense. Can anyone think of something obvious that I may have overlooked? Any help would be greatly appreciated.
I'm about done sinking money into this thing. I see a 5VZ in my future unless I can figure something out.
Thanks in advance for any assistance.

JasonYota

iTrader: (1)

Is the miss on one cylinder or does it jump around?

scope103

To answer this question, you can put the inductive pickup of your timing light on each plug wire, looking for a missing flash. You can use a "stethoscope" (the core of a roll of paper towels works pretty well) so see if you can localize the sound.

gmc

I haven't been able to pin point it. I started pulling plug wires this evening but got sidetracked. I know 2, 4 and 6 are firing. I'm going to try to get to 1,3 and 5 tomorrow. I've been wondering if it could possibly be a loose connection on one of the injectors. If one of those cylinders is dead I guess I'll be pulling the intake again.

Gamefreakgc

Compression check! You can get it done fairly cheap if you don't have the tools. That way you know what cylinder is giving you grief. If it all checks out ok, you know it's a fuel or spark issue.

gmc

Update from tonight. I pulled each plug wire while the engine was running and all cylinders produced a rougher idle. I also hooked the timing light up to each lead and there is a pretty consistent pulse pattern on all 6.
I pulled the plugs and all are nice grayish white. May be running a little lean if anything.
Compression is as follows:
1- 195
2- 187
3- 198
4- 192
5- 196
6- 194
Readings are a little high (could be my gauge) but all within 14 psi per the manual. There was no leak down in any of the cylinders.


The drivability is getting worse. In high gear any throttle input causes a bad stumble and hesitation. (I only get up to 50 mph on my daily commute) I now have to down shift to 3rd or 4th at speeds below 35. Used to be able to slowly accelerate from 25 in 5th gear with no problem.

It has a rough idle and really starts stumbling with the throttle blade held barely open. It will stumble a little if I barely crack the throttle but if I rev it up and bring it back down to the same spot, it gets a lot worse. Once you get past that spot it revs smoothly.
Any ideas? Thanks again.

JasonYota

iTrader: (1)

Is it throwing any codes?

gmc

24
Had 24 and 31 before I reset it last night.

JasonYota

iTrader: (1)

Checked fuel injectors? That's what code 24 is correct?

gmc

24=Open or short in intake air temp. sensor circuit for 0.5 sec. or more. (THA)

Punchy

Bad AFM or the wiring harness has breaks in it.


Just and off the wall test to try,,, Swap out the EFI relay with say the starter or AC relay.
Also while the engine is running, Move the engine wiring harness around in different spots and see if the engine idle changes its miss (especially where it crosses over to the engine at the battery, But some models its elsewhere)

Ca$ey

I'm battling the same issue I think. Except when my truck reaches normal operating temp the problem goes away. If you unplugged the vafm it is probably where you got your code from the open intake air temp circuit. I forgot to plug mine in and got the same code. Good luck if I make any headway with mine I'll be sure to let you know.

gmc

I pulled the intake again to check over my work and put my mind at ease. I also replaced the NGKs with some Densos and filled the tank up with some high test. I've noticed a slight improvement but the problem still persists.

After resetting the ECU, all codes were cleared and no new ones have appeared.

The truck fires up at the first hit of the key and runs great when the revs are up, but at part throttle and light throttle under load it still bogs and stumbles (does it when cold or warm, but seems to be worse when warmed up).

My mind keeps going to the TPS, at least for the drivability issues. Is it possible for it to have a bad spot and still test good?

Ca$ey: good luck and please let me know if you come up with anything; I'll do the same.

Gevo

gmc, I was also thinking that a tps MAY create such symptoms if bad.

It's hard to test a single or more bad spots because most multimeters are not quick enough to react when testing resistance as such to show you a dead spot in the TPSs' range.

Have you checked for vacuum leaks??

gmc

I can't find any vacuum leaks and the engine pulls a steady 18 inches at idle.
Part of me thinks it may be the TPS but then I think it has to be something simple that I'm overlooking.
All tune up parts are new, new TB and plenum gasket, FPR, rebuilt injectors, new fuel hoses, ECT sensor and any cracked or brittle vacuum lines have been replaced. TPS and VAFM tested fine.


The best way I can think to describe the problem is imagine being in 5th gear at about 20 mph and trying to bury the throttle. It bucks and stumbles that way when cruising or trying to accelerate when the revs are below 2k.


In neutral it does it if you hold the revs anywhere between @1200 and 1800. If I stab the throttle in neutral, it revs with no hesitation at all. The problem only seems to happen at part throttle or during slow acceleration.

Ca$ey

Ok here's what I've found. Since my troubles seem to go away as soon as my engine reaches peak normal operating temp I thought I would try replacing the coolant temp sensor. It didn't help but for $15 I figured I'd try. Tonight I was messing with it some more and I thought I could hear a vacume leak so I started feeling around trying to muffle the sound and decided it was coming from the egr valve. So just for grins I disconnected the vacume line from it and pluged it. Voila,no more stumble! I know it's not a fix but at least it's a step in diagnosing this problem. I wasn't able to drive it tonight but I put it in gear and slowly accelerated with the brakes held and it would easily spin the tires. I'll go test it tomorrow after work and report back. Hope this helps you.
Sorry for the long read!

gmc

Bringing this back to the top. I swapped the TPS, drained the gas tank and replaced the fuel pump sock last week. After swapping the TPS I rechecked the timing; when te1 and e1 are jumped shouldn't the idle drop and timing retard a few degrees? My idle doesn't change and timing advances a few degrees. I also noticed the timing mark seems bounce slightly back and forth a degree or 2 every few seconds.

On Saturday I took it out and it stumbled once and then ran fine the rest of the day. It felt a little down on power but a lot smoother and more responsive. I drove it to work this morning and it was back to the same. On the way home this evening it got worse. Any throttle input in any gear was met with a stumble and hard bog.
If I pushed the clutch in and rev'd it up it would clear up until I had to change the throttle input.
After stopping and restarting the truck, I ran it up to about 5k between shifts until I got it up to 50 mph and it ran fine the rest of the way home. It seems like the easier I drive it the worse it runs (it's 25 mph for the first couple miles leaving work so I can't really get the rpm up).

Anyone have ideas other than what's already been covered above? There are no codes, the TPS and VAFM test fine, there is good fuel pressure. Anything simple I may have overlooked?
I had the problem before I swapped the injectors but it has been worse since. Any chance the rebuilt injectors could be the cause? I would think injectors would cause a constant problem not just a part throttle issue.
I'm about done wasting money on this thing. I can't believe I'm considering an engine swap on a Toyota with only 79K miles on the clock. Any assistance would be greatly appreciated.

gmc

I did a little more troubleshooting today. After testing the distributor and coil I actually found my first discrepancy. The pickup coil, air gap and secondary windings are within spec but the primary windings were not. Max resistance should be .55 cold and .66 hot- mine tested at 1.5 when warm. Book says to replace if not within spec, but could this really be enough of a difference to cause my problems?

RJR

Could be just lead and connector resistance in your multimeter wires as well. Touch the tips of your two multimeter leads together and note the resistance reading. Subtract that number from the reading you get when you measure the coil and that will be closer to the real resistance of the coil.

crash813

Hey GMC, just so you know, I'm in the exact same position as you....almost identical except I have a hell of a lot more miles then you!

Some stumble at idle, but especially bad under load....ie. mines and auto so when you put it in gear, it misses bad. Use to just be at idle, then idle and low rpm, now it can be almost any rpm....so its getting worse....and frustrating.

I verified timing, replaced almost all vacuum lines, plug wires, cap, rotor, removed all EGR and PAIR equipment, testing TPS and AFM, flushed and filled coolant several times. Plugs are only 6 months old but thats next. Then checking the coil. If not, going to the junkyard to get another AFM and swap. Mine seems to have a voltage leak in the housing which i don't know if its normal or not but I swinging at anything right now. The wire, cap, and rotor seemed to help a little so I'm guessing the air/fuel mixture just isn't right.

Hang in there!