93 rough idle piss poor performance at lower RPM

[Karl Pollock]

Seriously, where do I start? First a little background. Got the rig 3 weeks ago. Low rough idle when cold very low power at lower RPM range. Seems to idle better when good and warm. What I've done so far. Cap & rotor new plugs and wires air filter fuel filter. Now when started cold I have the same idle issue along with knocking and a little bit of backfire and nearly dying when throttle is applied. I can cough it around the block a few times until it warms up then it runs like it did before. My goal was to improve the performance. Looks like I did the opposite. I'll add that I'm under no delusions here, the truck is 22 years old and needs a lot of work. But it seems I picked the wrong starting point. Any pointers/mockery would be apriciated.

 

[Kfire01]

Do you have a factory service manual? It is the best way to systematically troubleshoot these rigs. Try a search, there are a couple of links on here to obtain one. Invaluable.

 

[Karl Pollock]

I do have the FSM. I guess the shorter question is. Is it be easer to chase this around, or pull down and rebuild the entire intake assembly?

 

[baldilocks]

Like he said above, a FSM is mandatory if you are going to own one of these old trucks. You will learn a lot by doing you own work per the manual. Your problem has to many variables to diagnose here.
You bought it three weeks ago. How did it run when you bought it?

 

[Karl Pollock]

Marginally better than it does now. I think I'll go back to the beginning and do a compression check, see what that yields then follow procedure from there.

 

[Brent Preciado]

Hi there, my 94 has the EXACT same issue with poor idling and bogging when cold and fine when warmed up. Started after a trip up the back side of Yosemite at over 9000ft elevation after zero issues and 2yrs of ownership. I replaced the same items as well. Hope this thread keeps going the manual is just frustrating at this point.

 

[elhombre]

Check your timing per the FSM instructions.

 

[toyotaspeed90]

Check your base timing

Check for vacuum leaks - easiest/simplest way (once it is idling), spray over specific areas of the intake with carb cleaner - if the engine rpm increases at any point then there is a vacuum leak - start inspecting. Be careful not to hit "hot" areas such as the exhaust - carb cleaner is flammable.

The AFM (air flow meter) is a huge component of the early Toyota EFI systems - and any vacuum leaks after the AFM (as in - between the AFM and the engine, including any other kinds of vacuum leaks) can cause a rough idle, low idle, poor performance, engine dieing, and more. Pull/check the tube between the AFM and the engine for any signs of a hole, crack, or leak. Upon reinstall make sure the clamps are tight.

Check the TPS - the car will run fine with it unplugged - it may throw a code, but if the TPS is causing the problems then unplugging it will, essentially, have the ecu ignore the TPS and run without it. Do not remove the TPS and reinstall unless you have the proper tools to verify it is calibrated accurately (other companies don't require a calibration, like GM, and they can be removed/installed at a whim, but Toyota requires them to be 'set'). Unplugging and starting will really be more of a simple diagnostic to see if/how different the vehicle runs without it plugged in.

 

[Kfire01]

Bench test the AFM as well. One that is out of spec will certainly cause idle issues. The test is in the manual and fairly straight forward. Just be careful unplugging it. Don't remove the screws at the harness. You won't like what happens.

 

[Karl Pollock]

Thank you everyone for the advise. I have hooked up an FSM for the thing and think I can make some head way this weekend. I'm guessing I need a better quality volt/ohm meter if I'm going to be bench testing electronics. Switching from carbureted projects to EFI has been a humbling experience.

 

[Timpanogos Slim]

Vacuum leaks, intake tube leak, idle air valve. maybe timing too.

The fast way to test the vac and intake tube leaks is by spraying something flammable in the area where you are searching for a leak and listening for the idle to increase. A propane torch that isn't lit is one good option, lots of people use cleaning solvents, etc.

When testing the air flow meter, beware that the 2nd page of that section of the FSM has an incorrect diagram. The first page has the correct pins listed.

Also do not remove the screws on the connector. You have to remove the wire clip to pull the cable. Removing the screws often results in a ruined meter.

There are around 7 or 8 feet of vacuum lines on this truck. Some of them are under the upper half of the intake plenum. i believe 3mm is the correct size, 1/8" probably Close Enough.

As for the IAC valve, I think the FSM is probably correct when it says to remove the throttle body from the intake to remove the IAC.

 

[Timpanogos Slim]

For what it's worth, my '94 had similar symptoms when i got it, but intermittently.

At this point all but a few of my vacuum lines under the intake plenum are replaced, and i occasionally have low idle that is probably the IAC, but i haven't removed it yet.

I also found that the big wire loop hose clamps on my air hose were not able to seal the ends very well, and switched to t-bolt band clamps.

90's engines are all sort of a godforsaken blend of not-very-smart computerization and stone-age hydraulic engineering. They are touchy and they don't talk to you when you need more information.

The vacuum system is a very big deal. Some people have found hard vacuum lines clogged, and have been able to clear them with a length of guitar string or similar.

The engine sensors, such as they are, are also a big deal. Since you have a '93, check with toyota to see if the o2 sensor recall has been performed. 93/94 o2 sensors are different from the other years, and there is evidence that universal bosch or ntk sensors don't work well on these years. The dealer and denso parts (same parts) are the only ones that work reliably, and they are very expensive.

When i bought my '94 it idled rough and low, and i couldn't get it over 70mph. After a lot of little things (and the damn o2 sensors), I still have intermittent low (but smooth!) idle, a lot more power, and the gps says my top speed is 92. MPG is still 10 city / 14 highway.

 

[arcteryx]

as mentioned, check your intake tube. these are prone to cracks given their age. having unmetered air will give a rough idle, poor performance at low RPMs, it will settle down at higher RPMs. Don't be surprised if you need to replace this. be careful removing it too as they are brittle.

When removed, you may as well clean out the Tbody. 4 bolts remove it. Get some cleaner and a toothbrush. Easy to do.

Next would be the AFM/MAF as mentioned. FSM tells you how to test for this, but will take some time with a digital meter.

A compression test is always good for a baseline. If it's knocking, check timing for sure.

Report back after these are done.

 

[Karl Pollock]

Ok I replaced the fuel sock and cleaned up all of the connections and replaced all gaskets at the tank. I know there's a resistor and relay there too but I'm skipping them for now. I have removed and cleaned the air cleaner assembly inspected the intake tube, it's good, and bench tested the vam/maf unit. The results look bad. High on all counts. Please check my math. At rest .942-.961 k ohms fully open 1.75-1.78 k ohms. I believe the s equates to 942-962 and 1750-1780 ohms. FSM states 200-600 and 20-1200. The test was done with a calibrated meter I have no doubts about it's accuracy. Can these be repaired or re calibrated?

 

[Kfire01]

I found a place on the Internet that claims they can refurbish them, search "refurbish afm" though I can't testify to their success. You could also check classifieds here and see if you could find a used one.

 

[Karl Pollock]

I got bad numbers using the FSM bench test diagram. Using the in vehicle diagram I got everything in spec. This goes along with the error @Timpanogos Slim mentioned earlier I am going to say this unit is good and continue. New figures are: closed 257.33 open 49-50.

 

[Karl Pollock]

Compression test results are stellar. These were achieved with 6 strokes on the starter. Each cylinder was tested twice yielding the same numbers. As good as these look I'm forgoing the wet test and saying the HG and valve train is doing well. I plan to send oil out for testing when changed.
1: 172.5 lbs
2: 170.0 lbs
3: 180.0 lbs
4: 172.5 lbs
5: 172.5 lbs
6: 170.0 lbs
After this I removed the throttle body, pics below. Although it's real dirty I see no evidence of a vac leak on the gasket face. What do you folks use to clean these with, stodard solvent, moon shine, the tears of your neglected spouse?

 

[Karl Pollock]

If any one is still reading. This is pretty funny in retrospect. I went to give the t-body a good and thurough cleaning tested all vac ports and holes for function. Found port "E" plugged solid. Got some music wire, jamed it in there and broke it off, ****! By this time I'd had a bit to drink so without hesitation I cut the tube off at the elbow 1/4" above the casting and found 2 more pieces of wire in there for a total of three! Not the first rodeo for this t- body. I picked the wire out after mixing another and went after the port again still no luck. Another cocktail later, **** it get a drill bit I'm a machinist damn it! You guesed it, bonus hole in t-body. No worries though, I've got a whole intake coming soon thanks to @slow95z. Valve cover assembly is next, I'll do this one sober, or not…

 

[Karl Pollock]

They were seals in the literary sense, they were round and once made of rubber.

New seals and gromet installed. I haven't dug into it yet, but IMO the state of those seals and the pcv gromet could constitute a huge vac leak. The finished product pre installation.

 

[slow95z]

if you need any help with it let me know