Since the weather got colder I noticed the car has gradually lost power and what started as a slight stumble on acceleration has turned into a loss of power...like only running on 6 or 7 cylinders and small vibration. I initially had a code 33, but it was for the MAP, I cleared it and it has not come back yet...maybe it is bad and not throwing codes? Here is what I have checked and replaced so far. No SES light or Check Engine. Car starts and idles fine.
Checked Fuel Injectors Ohms all of them are 17.1
Checked Fuel Pressure and it is 43 running (I have a AFPR set at 50 not running) been that way since last summer.
Checked spark plugs (replaced over the spring) they are all still white
Check Cap and Rotor (clean, also replaced last spring with new Dizzy).
Checked TPS voltage and it is .54 closed
02 sensor was replaced last spring
Replaced IAC and Engine Coolant Sensor last summer.
Replaced Ignition Coil and Spark Plug Wires (no change)

Could the ignition control module be causing a stumble? Most of my experience with ICM is that they intermittently fail until one day the car just won't start. (I replaced the Dizzy complete last spring).

If I'm not imagining things, the problem seems to go away when I'm near empty, when I fill up the tank it seems to have issues. I need to verify this though and run the tank down until empty again to see if I'm not mistaken. The gas cap is holding pressure too, when it gets down to E and I twist the cap off slowly it releases pressure.

Very puzzling as to why it's lost all power and has no pickup during acceleration...seems to be shifting weird as a result too, all of the vacuum lines are good and connected.

 

While driving home tonight the car got really hot and it is cold out. I'm thinking maybe one of the two cats is clogged? Also can the Charcoal Cannister cause this type of problem? Anyway to bypass it for testing?

 

What's your fuel pressure while driving? Checking it in the driveway when fuel demand is low is one thing... but when demand is high, you need to make sure the fuel system can maintain the pressure.

 

Yeah good idea I will try that tomorrow and circle back on this thread. It should be like at least 43 psi at WOT or hard acceleration right?

 

Some thing like that.

But you're looking for a precipitous drop in pressure from what you're describing...

 

Investigate that. IAT sensor giving you trouble?

 

Fuel pressure is rock solid, I actually backed it off a bit from 50 psi at WOT to 45-48, it idles around 39 psi now. Not a hiccup with fuel delivery. It is still pretty cool outside, I let the car warm up to 220, took it for a short drive (1 mile), came back and parked it in idle. I rev'd it up to 2500 rpms for about 2 mins in Park, and the temp gauge climbed up to 260 really quick and the secondary fan kicked on. I seems I could do that in summer time and it would take a while for the temp to climb. This is what's leading me to believe it is a clogged cat. It is also shifting very hard, example when I take off from a stop if I give it the gas it will kick down abruptly, then upshift with a jerk and it is also revs fast. It will rev easily to 5500 rpms then upshift, but it also upshifts very hard at a lower rpm when I start to back off the pedal a bit as speed increases. All of the injectors are ticking away I can hear and feel them when I touched.

I'm not sure if the OBD 1 would trigger a code for IAT sensor failure, I have no codes at all. If the IATs were sensing hot it would dump more fuel I suspect.

Could a knock sensor failure cause this?

 

If the sensor is sending false readings to the ECM, causing the ECM to retard the timing, you could get loss of power.

If the IAT is actually sending a signal, albeit wrong, the ECM won't throw a code over it (unless it's outside the expected resistance range). But it'll simply react to the signal and adjust fueling accordingly. If you want to test the OBD1 response to an actual failed sensor (where the thermistor has gone completely open circuit), simply unplug the sensor.

But you can test the sensor with an ohmmeter. Videos all over youtube probably. Search on coolant sensor too since it's the same sensor being used in both applications.

 

I installed a new CTS last summer, and still have the old one that was good as the new one didn't fix the issue I was having. So I read in my Chilton manual the table with the ohm values for checking. As far as unplugging the IAT sensor the car should run OK enough to tell if the issue goes away? I'm pretty sure I can reach the connector to unplug it, but it is unlikely that I will be able to plug it back in without removing the plenum, but that is no big deal if it fixes the problem as I will need to pull the sensor anyway, which means taking the plenum off. Thanks.

What about clogged/plugged cats?

 

I don't know how it'll run with the IAT sensor disconnected.

Though you might disconnect the O2 sensor and force open loop operation. Whether it improves or not could give a clue what the problem is.

As far as the IAT, the ECM will adjust fueling based on IAT whether in closed or open loop. If the performance improves with the O2 disconnected then its unlikely the IAT sensor.

Also if it improves then it tells us that something is causing the ECM to alter the fueling for some reason.

 

Disconnected the O2 sensor after the car warmed up, drove about 3 miles and the only change is that a SES light triggered...which was expected with the O2 disconnected. The problem still persists, thinking it is a clogged cat, car otherwise starts, idles, and doesn't stall when dropping into gear or taking off. Only under acceleration is it stumbling and sounds weird like straining. I going to remove the mid pipe and check the cats...before this started happening there was an extremely bad smell coming from the exhaust, I started using Chevron with Techron and thought it was the additive burning off...might have been the cat material burning if it got clogged up on the exit.

 

An excellent thought and absolutely worth checking.

 

If you wanted to check the IAT (nothing we've done so far as ruled that out)... Looks like you've already replaced the CTS.

https://tpiparts.net/90_92_speed_density_sensors/ (scroll about half way down)...

Whatever the local ambient temperature is in your area, get a resistor of that value (probably quite a bit cheaper than a new sensor). Unplug the IAT sensor and stick the resistor across the terminals. If you're 86-92, then the IAT should be the same as the CTS according to that link.

During initial start up and before the engine builds up any serious heat, the value will essentially be correct.

Also, check your TPS voltage through the entire throttle movement, not just at idle. Ensure you're getting smooth transitions in the voltage. Should be linear.

If your car is a MAP car, you can use a hand held vacuum pump and an voltmeter to see how the MAP sensor is reacting to changes in pressure. Again, voltage changes should smooth and linear. That link above also has the values for the MAP sensor. You'll just have to gain access to the terminals on the MAP sensor while the plug is still connected.

 

I swapped in another TPS I had and measured the voltage and it is the same as the one I had installed nice and smooth from .54 to 4.63 at WOT. To eliminate the possibility of having a clogged cat, I swapped in a spare dual cat mid pipe I had and the result is the same. I'm starting to wonder if maybe the EGR I installed last summer somehow went bad or the something is clogging the fuel injectors. Whatever it is, it is causing the upshift to hold sometimes/shudder and then upshifts really hard. Can the EGR be disconnected in place to test if it is the issue? If I have to take off the plenum to change the EGR I might as well take off the fuel rail and change out the injectors, kind of a pain in the *** if it is doesn't work. I can hear and feel every injector clicking and they all ohm'd out to 17.2, so unless they are completed clogged somehow I can't think of anything else that would cause this issue...ECM perhaps?

Something else that I have noticed is that it has been getting horrible gas mileage. I filled up the tank it is at 1/4 tank and the trip meter is showing 130 miles. I always reset the trip meter when I fill up and the norm is 240-250 miles per fill up. So something is effecting the MPG as well.

 

You said in the first post that the problem goes away with low fuel levels... is that still actually the case?

May want to check your map sensor. If something is wrong with it, it could be telling the ECM you're constantly at WOT, which would definitely cause the car to run very rich... which would lead to bad gas mileage.

 

It's getting down to low fuel level now, the problem is that I replaced the MAP sensor after I had a SES code light that reset after start up. I pulled codes and 33 Code flashed after the normal 12 code. I disconnected the battery after replacing with a new MAP, and the code didn't come back but the problem persisted. I put back in the old MAP and the problem is the same. I am going to recheck the CTS and IAT next. The CTS is fairly new, but I have the old one which is still good just tested it with my ohm meter and it was 1.6 at 100* F.

 

That's 1.6kOhm, right?

 

Yes

Just to test something out I disconnected the EST connector and the car seemed to run better. I also noticed some oil underneath the cap when I checked the rotor and ICM, I cleaned the cap real good and rotor perhaps some oil got on the pick up coil or the ECM is having some issue.

 

you should have a wideband as it would simplify diagnostics and visually indicate a whole slew of systems are working or not instantly

 

You're right but I don't, so I have been just reading ohms, voltages, and pressure. I have another distributor that is good I may throw that in to see if it fixes anything. Next I will check the Knock Sensor to see if it is causing a retard in timing.

 

Well this is embarrassing but I'm not going to spare myself and not update this thread with what I found as there are too many threads on this board where there isn't a conclusion or update.

In my troubleshooting efforts I became convinced that it had to be either a bad ECM or long shot the ignition pickup coil in the dizzy (even though it is only 8 mos old). I still had my old dizzy that I replaced last summer with a new one thinking it was bad (it ended up being a Made in China Ignition Coil that was intermittently shorting). Before I took out the new diizy I went ahead and marked rotor position using a long pencil from the rotor pointer slot to mark the firewall. I installed my old dizzy and the car started up fine, dropped it into gear and when I tried to move it just stalled. So I thought what the hell now?? So I broke out my timing light and disconnected the EST and to my complete surprise the base timing was 6* ATDC, I hooked back up the EST and the timing was 8* BTDC. My initial thoughts were how the hell did the new dizzy I installed last summer get to this position????

Last summer after replacing the dizzy and ignition coil and set the timing to 6* BTDC and the car had been running fantastic all year until this December. However the temps dipped down into the low 20* which isn't the coldest I have seen in the foothills (seen 17*) but the dizzy must have slowly been turning and as a result the timing getting retarded more and more until the point where it was causing a major issue. This also explains the very poor mpg I was getting per tank.

Using my old dizzy I reset the timing (it's actually at 8* now, I know 6* is correct but the car just runs better at 8* base timing) and the car is running perfect again. I need to pull the old dizzy out though and put my new one back in as the old dizzy pickup coil was rusty looking...obviously it still works.

So anyway lesson learned always recheck the timing even if you think it is good because there were no other changes to the dizzy. In my case it simply backed off when it got cold...shrinkage of metal allowed a little more clearance for the hold down bolt and retaining clip allowing it to meander and retard timing over a month of colder weather.

One last update, although the timing some how was way off, after I reinstalled the newer distributor that I swapped for my old one, the issue came back with the misfire. Looks like a bad pick up coil in the new Autozone Distributor is the issue as I had swapped ICMs, Rotors, and Cap with the old one. I ended up putting the old dizzy back in and the car is running fine again. Frustrating when new parts go bad, my old dizzy has a rusty looking pickup coil and it is still working fine, I just replaced the ICM, Cap, and Rotor in it.


Closing the loop on all of my threads regarding misfire under acceleration. The problem ended up being two fold.

First- my distributor hold down bolt loosened up and the distributor rotated clockwise and my timing was retarded at 4* ATDC. I re-timed it with a light and this resolved 90% of the issue. However something still felt off...down on power and not accelerating as it should from 2500-3500 rpms and really lazy to 5500 rpms (2nd -3rd shift).

Second- When I went to find TDC for re-installing the dizzy, I noticed the #1 plug had some insulation glazing, most likely from driving with retarded timing created some heat or something and glazed the plug. I never really have seen this before so I initially put the plug back in. After more thinking about it some more, and being convinced it was a spark issue, I pulled all of the plugs and they looked the same. I replaced the plugs and the car now does not miss.....imagine that. All is good. Thanks for everybody who chimed in.

Issue resolved.