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Rough idle/stall issue

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ljense8

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I just recently bought a 91 Blazer. It runs real rough when cold, and gets better when it warms up, but it still stalls when I stop if it's in gear. I haven't had time to check much, but if anybody has any ideas of what would be the simplest/cheapest things to check first let me know. BTW, it has a TBI setup on a 355(bored over 350) with mild cam. I suspect something with the TBI spacer, or low fuel pressure. Thanks.
 
Assuming it ran fine before, like after motor was installed and tuned with the cam and blah blah. Check fuel pressure, should hover between 9-12psi. I would try and make it stick around 13ish though, or 12 depends on the motor. Second, after that, assuming the plugs, wires, cap, rotor ect is all good, check the operation of the idle air control valve, its on the right side (passenger) of the TB. Its a plunger type thing that lets more air in when the blades close, sorta like a choke. Also make sure no VAC leaks around TB, you can use a length of 1/4in VAC hose and put one end in your ear and the other around the intake and ect, you will hear a hissing if leak is present. DONT get caught in fan, I know it sounds dumb, but everyone that got killed by a fan said or thought the same thing. Lastly, check the coolant temp sensor, on the intake/thermostat housing. If it has a check engine light it will be easier to diagnose. A scanner will tell you IAC position, and how it moves, TPS positon, Coolant temp, ect. Might be very helpful if you can get ahold of one, but unless you have a mechanic buddy it could prove to be difficult.
 
Thanks for the tips. I wont be able to look at it this weekend since I will be out of town(driving my wife's vehicle), but I will update here when I do get a chance. Also, you are right, it sounds kind of dumb about the fan, but dumb stuff happens, so thanks for the warning. I've seen enough dumb injuries happen either on the job or working on cars to be offended by warnings like that.
 
ljense8 said:
I just recently bought a 91 Blazer. It runs real rough when cold, and gets better when it warms up, but it still stalls when I stop if it's in gear. I haven't had time to check much, but if anybody has any ideas of what would be the simplest/cheapest things to check first let me know. BTW, it has a TBI setup on a 355(bored over 350) with mild cam. I suspect something with the TBI spacer, or low fuel pressure. Thanks.
Click to expand...
might check for sticking egr valve,,,fairly common and will cause both rough idle and stall when you put into gear
 
Easy stuff first, check for underhood hose condition, electrical connections, good plugs/wires/cap/rotor, then delve into loose bolts (buddies '91 had loose intake and Throttle Body bolts)/vacuum leaks, especially the TBI base gasket. EGR is as mentioned a common issue as well.

If you can get your hands on a real scanner (one that shows engine temp, etc) that would help diagnose a potential coolant temp sensor (CTS) problem. Alternative is an ALDL cable, laptop, and software like winALDL or Tunerpro RT for viewing engine data.

Whatever you do, don't spend a bunch of money guessing on parts, it will quickly add up to more than what the proper diagnostic equipment would cost.
 
I am looking at the laptop/ALDL cable option. Will this just let me see engine data or will I be able to actually read codes as well? I have an OBDII scanner that I use quite often for troubleshooting(codes and live datalogging) on our newer vehicles and would like to have the same ability for my Blazer without buying a $400 OBD1 scanner.
 
OBD1 codes can be read with a paperclip.

I can't recall if winALDL will show codes, but I know tunerpro will, just not sure if the codes would be "sent" through the cable in the TBI setups. I suspect yes. If nothing else the ALDL cable is fun to play with, and moderately useful for monitoring the various sensors. Unfortunately with TBI the data transmission rate is horrible, so it's not nearly as "real time" as some of the other ECM's GM was using at that time, like the TPI ones.
 
Thanks for the heads up. I have been out of town so I haven't been able to figure anything out, but the more I ponder the issue I am beginning to suspect it was either not tuned or tuned poorly for the new cam setup. The other thing I will check ASAP when I get back home is the timing. I plan on a new fuel filter also. The filter on it looks like the original and it has 130k miles on it. Thanks for the help guys, it's driving me nuts not being able to get in the garage and solve the issue, but I should be able to get at it tomorrow some time. I'll update what I find. I will also be starting a build thread soon when I get some pics.
 
Used the paper clip method and got a code "15"(Coolant temp sensor low) so I did some checking. I have a mechanical temp sensor hooked up, so the computer is not receiving any temp signal. I am guessing this has something to do with the issue. I will be going to pick up a new electronic sensor Thursday. The wire is still in the engine bay, just kind of flopping around so I tied it to the wire bundle it runs with until I get the new sensor. If anybody knows a reason not to get the new sensor and just leave the mechanical in place let me know. Thanks.
 
The CTS is a two wire sensor. The gauge sender is a single green wire.

CTS is going to be in the intake. If it's present, while cheap, I'd still want to verify it was faulty, vs. the wiring or connectors.
 
Yea and its a pain to get. They always bring out the 1 wire one for the gauge. The 1 wire is below exhaust manifolds on driverside. The 2 wire is on thermostat housing or the intake.
 
They should be named a sender (for a gauge), and a sensor. I know, parts counter guys aren't always the best, and the way their systems have them tagged are also sometimes the problem.
 
I will definitely check the wiring and plug on the sensor. I didn't realize there are separate sensors for the gauge and CTS so I will see what I can figure out. In the meantime I found a leaky vacuum tube fitting. After repairing that, it has smoothed out some, but still a little finicky when cold starting, and it also seems to have developed a cold... it's backfiring like crazy when cruising or decelerating. A pretty thorough tune-up is in order this weekend(if time permits; I have three kids and a full time job battling for my attention) along with a new CTS(if it is confirmed to be faulty). Will post updates as I figure things out. Thanks.
 
BTW, just to confirm, I was in fact referring to the 1 wire sensor which has been replaced with a mechanical. My oil gauge has also been replaced with a mechanical unit.
 
Well if memory serves me correctly, if the CTS fails, or is unhooked, it for some reason automatically sends the ECM a -40* setting. Which is WAAAAAAAAAY cold, and it will cause all sorts of run problems.

That being said, the backfiring sounds kinda like an EGR being all douched up, with the engine running, grab under the EGR and press the diaphram "up" into the housing itself, and the truck should start stumbling or drop the idle noticeably. For some reason these little POS's dont toss codes often, atleast on mine, and neither does the VAC controller device thing that the EGR runs too.

But if its tossing a CTS code, and your sure its not the connector or associated wiring, just toss a new one in it, unhook the battery for a min or two to clear codes, and hook it back up and fire it up and drive it for a bit and see if it improves. If not, I would try the EGR thing above, as you want to diagnose the problems, not treat the symptoms. So make sure its all codes cleared, then try the EGR and other related things.
 
This is one of those places where the ALDL cable or scanner is really handy. In about 5 seconds you'll know if the CTS is reading correctly or not.

Just like anything else of course, it won't tell you what the actual problem is, simply that there is some reason that sensor isn't being read by the ECM correctly...wiring, connectors, failed component, etc. can all cause problems. You seem to be on the right track though. :)

FWIW, the CTS' seem to be pretty robust pieces.
 
It appears I don't have an EGR valve. Also, what colors are the wires for the CTS? I found a 2 wire plug with black and yellow wires that seems to be in the right location, but isn't plugged in to anything. Actually, there are several wires throughout the engine bay that are just hanging loose. I am assuming/hoping most of those are unneeded after the engine swap. I also found an O2 sensor just under the header that isn't connected, and I can't find the wire for it. This is shaping up to be a little more work than I hoped for in the drive train department. Whoever did the engine swap, lift, and exhaust did pretty sloppy work. On a positive note, it's definitely providing a good learning experience.:dunno:
 
Ok, the O2 is a single solid purple wire coming out of the 1in loom behind the distributer, not to be confused with red wire with a black stripe. That red/black wire is a timing wire. Now yes, the CTS wires are black and yellow. If you are looking down at it, with the snaplock facing away from you, and the wires leaving the connector and comin towards you, yellow is on right, black to the left.

The O2 should be on driverside in the manifold.

Hope that helps, let me know if ya need more, a Haynes manual would be most helpful to you, as they have easy to read wiring diagrams that show wire color. Available cheap from parts houses
 
Also somethin else crossed my mind, if it doesn't have an egr, which on a SB is on pass side of the throttle body, it should have a block off plate. But the egr uses a vacuum regulator thing that may not have been capped off, you should check that as well
 
Yellow/black I believe is the correct ones for the CTS.

The "good" thing is that while it will take some work, I'm betting a ton, if not all of the problems, in driveability you have will be solved when you fix all the obvious issues. Much better than trying to track down an intermittent electrical problem!
 
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