My 95 starts fine but will idle rough , has flat spots on accel, seems worse when cold occasional backfire. so far I have ..

checked fuel pressure ..has just over 60 pds when key is turned on bleeds off slowly when key is off.I see no gas coming out of fuel pressure reg.or from any part of cpi
IS IT POSSIBLE FOR FPR TO BE BAD BUT NOT LEAK GAS INTO INTAKE??

tested tps and found all readings within spec
tested map sensor and found all readings within range
removed egr found no carbon deposits.. cleaned and reinstalled
removed upper half of plenum to inspect for leaks found no gas laying in lower half of intake anywhere
resistance checked plug wires tested good(low resistance numbers)
I installed a new coolant temp sensor since it was easy to get at with egr and plenum off ,have not started since I installed it.

Before I reinstall plenum any more suggestions?
Has new pump and tank assembly

Thanks in advance !

 

1995 has one of 2 different injection systems, and 3 different computer controls. Does your engine look like this?



Does your OBD connector under the driver side of the dash look like the top one, or the bottom one?

 

thank you for your reply I have not had time to work on my blazer but I did check this morning and the connector I have is the one of the shown in the bottom picture the engine does look like the picture you posted. At this point the issue I am experiencing is basically at cold startup the engine tends to surge and has a bit of a miss but when it warms up it runs okay but with a bit of a rough idle. I am wondering about the I at sensor...does not have a maf or mass.
Thanks

 

Your engine has CMFI, (Central Multiport Fuel Injection) with OBD1.5 engine controls. The PCM provides "P" code DTC's with 4 digits following, rather than 2 digit codes. The system is not fully OBDII compliant, and not all scan tools can cummunicate with your PCM.

You're right, it does not have a mass air flow sensor, (MAF). If there is a fault with the IAT sensor, there will be a DTC set in memory, and the check engine light will be on. Is the check engine light on, and have you checked for trouble codes?

First step is to check fuel pressure and leakdown. Most auto parts stores rent/loan the tester for free, and sell them for around $40. The test is quick & easy, only takes about 15 minutes. The results will tell us if the fuel delivery system is working properly. When you have a tester available, let us know and we can explain how to do the test.

 

I installed new fuel pump a month ago fuel pressure test results were in my 1st post
just over 60 psi with key in on position, pressure does bleed off slowly unless I clamp off return line to fuel tank.
Pulled plenem to inspect fpr and spider and found no leaks / washing anywhere.
have not found a scan tool to check codes yet..check engine light is on.

 

You can have the CEL code read for free at Oreilly's, or Autozone.

 

60psi is your regulated fuel pressure, and it's fine. The pressure must remain above 50psi for at least 10 minutes after the pump shuts off. If it fails one or both parts of the test, there is a problem with the fuel delivery system. It does not tell where the problem is, it only tells that a problem exists.

Restricting the return line is risky and not recommended, (the diaphragm in the regulator can be destroyed). If leakdown is excessive, which it sounds like yours might be, you need to check fuel pressure and leakdown at the fuel filter outlet. All fuel pressure and flow must end at the tester, (none allowed to the engine). While the pump is running, pressure must be ~90psi, and it must remain above 50psi for at least 10 minutes after the pump shuts off. Using this method will tell if the problem is in the tank, or in the plenum.