All,

This is my first post but have been "listening" for years and have used many of you all's advice. My 89 (218K miles on 4ZE1 engine) very quickly developed rough running and idling problems last week. In chronological order here are the symptoms and efforts thus far.
1) Engine would start fine but started idling roughly and slowly until warm.
2) Suddenly, and at freeway speeds the engine would not hold power under load; max speed became about 45 mph and engine seemed to be running on 2 or 3 cylinders.
3) I suspected vacuum leak so checked and replaced every vacuum hose visible with no change. Engine still runs rough although will run above about 1200 rpm after warm.
4) I verified spark and compression on all 4 cylinders although the 2 and 3 cylinders are weaker than 1 and 4.
5) I checked and re-set all 8 valves to spec. 2 were a little tight but it made no difference.
6) I verified operaton of the fuel pressure regulator per shop manual
7) I inspected the throttle body for obstructions, etc. and found nothing
8) I ran out of ideas and am writing this letter

I know there are a couple of vacuum lines underneath the air chamber and I verifiied their connection with a mirror but am wondering if there are other areas any of you know of that can typically cause such a leak or these same symptons?

Thanks for any help......Randy

Try changing the fuel filter. A clogged one can easily lead to the symptoms you are describing.

Ok, thanks, I didn't mention it but I did remove the filter and reverse flushed it with gas; I got a little bit of black residue washed out but was able to easily flow fuel through the filter, replaced and it didn't make any difference either. I haven't checked the fuel pressure yet either but that doesn't seem to fit the symptoms. I'll do that tonight.

I was hoping the key here was how much better it runs and idles when warm. Thought that might point to a specific component.

Randy

Check your ECM engine temperature sensor that threads into the underside of the intake manifold.

"There are two connectors on a bracket just behind the battery. The larger of these two has the 2 wires from the temp sensor in it. They are BLACK and GREEN w)YELLOW stripe on my 88 truck. The way the connectors are mounted on my truck, these two wires are right on the top of the connector. Simply unplug the connector and put your ohmmeter across those two pins (DON'T BEND THE PINS!!!!).
degrees F / K ohms
14 / 7-12
50 / 3-5
68 / 2-3
122 / 0.7-1
176 / 0.2-0.4" - Troopersphere

You could also try blowing out the MAF with compressed air and cleaning up the connector with contact cleaner. Reads like the air fuel mixture is wrong or inconsistent. A bad signal from either one of those could be the problem.

Ok, thanks for the advice and procedure for the temp sensor; I had been meaning to check that anyway.

Last night I did verify correct fuel pump, pressure and regulator operation. Then I put a vacuum guage on the intake plenum and verified vacuum consistent with the erratic "idle". Still won't idle below 1200 even when warm and seems to just shut off below that.

Then I shut off the engine and put a vacuum pump on the plenum fitting and am getting a BIG leak somewhere underneath the plenum. I did verifiy both vacuum lines on the bottom of the throttle body were attached but it was getting late so I'll put the guage on those tonight. After that I plan to pump some smoke through the air plenum to see if I can find the leak.

Thanks for the help,

Randy

CHECK THE LARGE BLACK -PLASTIC PIPE THAT GOES ACROSS THE VLV CVR TO THE THROTTLE BODY, FOR A BIG HOLE BURNED INTO THE BOTTOM RIGHT ABOUT THE VLV CVR , VERY COMMON ON HI MILAGE VEH, JERRY

Jerry,

Thanks for the info....I did have a hole in the MAF to TB hose....I patched it and that did fix the idle problem and it will now idle at 800 rpm or so but it is still only running on 2.5 or so cylinders. While idling I pulled all 4 plug wires one at a time and the #2 and #3 wires had almost no effect while the #1 and #4 would stall the engine.

I found about a .5 by 2 inch piece of plastic missing from where you said it would be and am wondering if that plastic may have been injested into the center 2 cylinders and is blocking an intake or exhaust valve?

I appreciate the help and understand that you all probably have better things to do....

Randy <img src="/forums/images/graemlins/barf.gif" alt="" />

From my reading, it seems that those holes are usually slowly worn or melted into the intake tube, so there would not be any significant debris to be sucked into the engine.

What were the results of the compression test? If you had one or more valves stuck open, you'd get zero compression on that (those) cylinder(s).

How are the cap and rotor? You could have weak or intermittent spark at those cylinders. Also, did you check the condition and gap settings of the plugs? Are the wires worn or cracked? The miss under load could definitely be ignition; also, worn ingition components can be affected by temperature (symptoms change as the engine temp changes).

I AGREE WITH ADRIAN HERE THAT TUBE WEARS SO SLOWLY , WITH THE HOLE THING THAT I DOUBT SERIOUSLY THAT WOULD BE THE PROBLEM, FIRST DO YOU HAVE A FACTORY CAP AND ROTOR AND WIRES , CAP AND ROTOR I HAVE SEEN MANY OF THEM KEEP THE ENG FROM RUNNING CORRECTLY, ANY GOOD SET OF PLUG WIRES SHOULD BE OK, DO YOU KNOW THE ACTUAL COMP, READING, TAKE A READING WITH A WARM ENG, BE CAREFUL GETTING THE PLUG S OUT , NOT A HOT ENG , PROP THE THROTTLE VLV ALL THE WAY OPEN AND SPIN IT OVER 4-5 REVOLUTIONS AND LETS SEE HOW CLOSE THE NO,S ARE , YOU COULD JUST HAVE A BURNED VLV, ALLSO CORRECT VLV ADJUSTMENT .. JERRY

Jim and Adrian, thanks for the additional comments. I was out of town until today but will run a compression check tonight and further inspect the ignition system.

I already adjusted all the valves. The cap, rotor and plugs only have about 10K miles on them and have been fine up until this problem. The plug wires are probably old enough to be suspect so I will replace them.

Thanks again for the help.